General Review: Here I will cover the basics of the codex that aren't exclusive to single units. Basically, it will be the Biomorphs, bioartifacts, Psychic Powers, Warlord Traits, and army special rules, etc...
First, Synapse: This is exactly what you think it is. No Eternal Warrior, no Feel no Pain, just Fearless. The only changes to this are that it may be extended by various upgrades... more on this later. A minuscule buff over the last edition just because of extension possibilities.
Shadow in the Warp: This one is new, and arguably improved. But, it is arguably nerfed. The basics are that Psychic units with 12" are at -3 leadership. Note that this is NOT just for Psychic Checks! So a Farseer led unit of guardians is much more likely to run away now, plus his Psychic checks are limited. So, Psykers are usually 9 or 10 Leadership. in old Synapse, a Leadership 10 Psyker had exactly 108/216 possible results end in Success, or 50%. Now, there are 21/36 that end in success, or 58.3%. Leadership 9, before, had 81/216 pass, or 37% chance of passing. Now, they have a 15/36, or 41.7%. So yes, our Psychic defense is not quite as good as before. However, factor in deny the witch and any malediction in Shadows has a less than 50% chance of passing. In exchange, we lower Psykers Ld all the time, which will help with morale, spirit Leech, etc. (Don't forget Daemonic Instability in CC! That's going to hurt.) Also, laugh at Grey Knights. Overall? Very minor nerf. And other benefits mean we really can't complain, especially seeing what happened to Eldar runes. We by far have the best anti-psyker of this edition, with only Space Wolves beating us. My question is, does Shadows extend with Synapse range? Nothing says it does, but it really should. Hopefully an FAQ will fix this.
Instinctive Behavior: Ok, here comes some bad news. There are three charts now, not two. Feed, Lurk and the new Hunt. We now have tables, which you roll on if you fall back. Here you are!
Lurk: 1-3 fall back as if you failed morale.
4-5 may move freely, but is not slowed by terrain. May only shoot if in terrain. Can't charge.
6 Same as 4-5, gains stealth.
Not great. A 1-3 blows clearly, and is a nerf. Yikes. However, the 4-5 and 6 are better than it was in the old book, as we now have better choice of movement and potentially stealth, plus cover benefits. Get that 6 and go to ground an objective in cover for a 2++! Still, the bad outweighs the good here, and thus there is a minor nerf to instinctive behavior lurk. This is the most important table in my mind, as our support units/objective holders use it.
Feed: 1-3 Inflict hits on unit equal to number of models, at average strength and Ap -, and can't do anything.
4-5 Can't run or shoot, must declare charge if in range.
6 See above, but gain rage.
This one is a clear nerf. Literally none of the results are as good as feed was last time, when it really didn't hurt. Sadly, swarm cannibalism is a sci-fi stereotype, so we must suffer. Limiting movement also sucks too. But, can move out of charge range oddly enough in the movement phase if you don't want to charge at least, you aren't forced to move anywhere. Also, one man units treat a 1-3 as a 4-5.
Hunt: 1-3 Go to ground immediately. If fearless, see 4-5.
4-5 Move as you want, must shoot closest unit if you can. No Run/Assault
6 See 4-5, gains preferred enemy.
Not too harsh. All of these are ok other than the loss to fearless really. This is clearly the best compared to the other two, where 1-3 is meh instead of murderous, and 4-5 is basically just "no fearless" while 6 is pretty decent. This is by far the best for objective holders... if only we had any of those with Lurk. Sigh.
Instinctive Behavior overall: Undoubtedly a nerf overall to us. This ain't pretty folks, as none of these are real benefits. Too many bad results to be happy. Basically, this means you MUST have Synapse, and it is more important than ever, or you will lose the unit's effectiveness.
Close Combat Weapons: Not a new rule, but a note. Each PAIR of weapons counts as one weapon now, so two pairs = plus one attack. Two pairs of Scything Talons, etc. Not bad, but many units lost attacks so this essentially does nothing. Also, can't use effects of two weapons. Only place this matters is can't rend while using a Bonesword basically.
Warlord Traits: These are a new 6th feature, that every new codex has gotten. As always, 6 results are possible...
1. Nature's bane: Every turn, a forest within 12" can become Carnivorous. (Every unit inside takes d3 Strength 5 hits in their shooting phase, on rear armor if vehicles.) This is literally awful. It applies to Tyranids in the forest, and we are more likely to go through them than our opponents are, as no one will advance towards us. If you roll it, don't even use it, unless it gets FAQd to not affect Nids like it should.
2. Heightened Senses: Warlord and units within 12" gain Night vision. Cool? Not useful as none of our warlords can shoot far, and frankly none of our other shooty units. Useful if you have Biovores nearby turn 1, but by 5 you are dead or up the board and close enough anyways. Very minorly useful.
3. Synaptic Lynchpin: Synapse increases to 18". Not great, but always useful at least. Good for Flyrants. Combo with Dominion and it's pretty good. For a free trait not bad at all. Best one so far.
4. Mind Eater: Receieve 2 points for killing an Independent Character in a Challenge. Pretty good, as it ensures no character will accept your challenge ever, just to not risk it. But if you catch one and whittle his squad down, he'll be all alone eventually and must accept. That's when this gets useful. If you see a Librarian on a Tac squad, it'd be well worth dropping your Flyrant down to assault.
5. Digestive Denial: One piece of terrain in enemy DZ has cover reduced by -1. Can't be used on bought terrain. This is pretty decent, as everyone will be hiding from us. Good for Ruins so our opponent can't have too much cover against the sheer dakka we can lay down at close ranges, so pretty good.
6. Adaptive Biology: Once the warlord takes a wound, he gains FnP at the start of his next turn. A bit annoying that it takes a turn to activate, but realistically not bad at all. FnP is super useful on MCs, so this is a trait worth having.
Warlord Trait final thoughts: I still prefer Strategic. Some of these are downright useless, but a few are kinda nifty. 1-2 are sad, but after that it gets a little more respectable. At the very least it's worth considering this over the BRB traits. I will be rolling strategic however.
Psychic Powers: Many people will complain that the loss of Biomancy is a nerf. They're totally right. But it happened, and now we must look at our now options with eyes unclouded by bias and disappointment. The past is dead, look to the future.
Primaris: Dominion: Extend Synapse by 6". Not great really, but handy occasionally. As a primaris it could be worse, but could be much better. Good if you happened to roll the Warlord trait too for a mighty bubble of Synapse. A lot of people are ripping on this because they look at it with a 5th edition point of view for Synapse. But, Synapse is way more important now than ever, so this is actually not bad. GW is literally telling us "get in Synapse or lose the unit". This is a way to save some units.
1. Catalyst: Give the caster and one unit within 12" FnP. Ok, clearly the best of the bunch. Roll it and grin, as that's two FnP units. Better than endurance honestly. Not much to say, don't drop this for Primaris.
2. The Horror: One enemy within 24" must take a pinning check at -2 Leadership. Situational. This is good against Tau, as many of their units will just drop to the dirt easily. Necrons, Daemons, etc, this is less useful and should be exchanged. Also combos with Shadows on Psykers, just saying. Again, pin Grey Knights with EASE. Many see this as Pseudo-assault grenades on Stealers, but we'll see in their entry.
3. Onslaught: 24" blessing, a unit may run and then shoot. Similar to before, but not great. However, it is useful if cast on FMCs, who gain a 2d6 run. Otherwise, it is useful to keep your bug guns moving, because frankly Nids must advance as a whole Synapse changes means no one can afford to be left behind. Cast on a fleet Dakkafex unit to get them moving fast while blasting away. Usually worth keeping.
4. Paroxysm: 24" Malediction, -d3 to WS and BS of target. I think this is the second best power. It helps a ton against Eldar and Tau especially, and it affects vehicles. Note that this can even target Flyers. So yeah, screw Vendettas and all that. It's best against non-MEQ units, where a 3+ forces BS 3 to snapshot. So Pathfinders, Broadsides, etc beware. If this stacks with itself (talk to local TOs/opponent) then it can combo to bring good shooting units way down, but I think it'd usually be better to hit two separate units. You should never trade this out.
5. Psychic Scream: 6" Nova, enemies roll 2d6+2 and subtract leadership, and take that many wounds. No armor/cover. This is pretty bad to me. Tervigons/Swarmlord/Zoeys will never be close enough to use this. On Flyrants I'd consider it on a low Leadership army, or heavily Psychic army (remember Shadows). Against Nonpsychic/high Leadership, consider the Primaris here.
6. Warp Blast: Same as before, 18" lance of Strength 10 AP 2. Not 1 anymore, 2. Can also be a Strength 5 ap 3 small blast. Warp Charge two, so Tervigons reroll this. I'll deal with Zoeys in their entry, but Flyrants should usually keep this. Good anti-flyer, good anti-vehicle, good anti-Daemon prince if it goes off. Only against a Green Tide type list would I potentially take the Primaris, as it does limit you to one other gun if used.
Final Psychic Thoughts: Almost all of these can be used, and the Primaris will cover up the mentioned weaknesses before. Yes Biomancy is better due to Iron Arm, but a lot of these can cripple opponent's effectiveness or just buff us in more Tyranid-y ways. A minor nerf, but not all is lost. They're definitely worth using!
Biomorph list: The gun upgrades will be covered in each unit's entry as it varies vastly by unit.
Close Combat Biomorphs: These are basically for Tyrants and Warriors/Shrikes/Primes.
Rending Claws: Costs one Hormagaunt: Good and cheap and deadly. Combo with Toxin Sacs for rerolls on lower Toughness guys and you're in good shape. You all know how to rend, but they are basic Ap. 5 which is cool too. Minor buff over last edition. If you want Warriors in CC, this is your best bet, but that's a separate issue. On Tyrant's there is essentially no point. (7/10)
Pair of Boneswords: Cost three Hormagaunts: So a bit more expensive, sadly. They are now Ap. 3, and cause Instant Death on a 6. Better chance of Instant death due to multiple attacks, but no more "ignore saves" and a points boost means this isn't a great option for warriors or Tyrants. Very cost prohibitive. (3/10)
Lashwhip and Bonesword: Costs four Hormagaunts: Again more expensive, bonesword is same as above. Lashwhips no longer reduce opponents initiative, but instead raise the wielder's by 3, so worse for assaulting into cover or for supporting other units like Lashguard with the Swarmlord. Again, this is costly. But on Tyrants, it's the best CC you can find if you choose that path. He'll rarely strike last and will kill many things he touches. (5/10)
Toxin Sacks: Costs two Hormagaunts. A good upgrade, it gives Poison for that re-roll to wound. This is a good choice on any Monstrous Creature you want to go into CC. However, not many of those are viable. (6/10)
Adrenal Glands: Costs three Hormagaunts. More expensive than before? I ain't even mad. At 5 more points, it gives fleet in addition to Furious Charge. This is huge for basically any MC that didn't have it before, so everyone but Trygons. If you can spare the points, (and I think Nids will do best bare-bones) then this is the upgrade to buy. On Carnifex it's a massive difference maker, even on shooty ones so they can get close, charge late game, and run with Onslaught. (8/10)
Acid Blood: Costs three Hormagaunts: If an opponent wounds you in CC, he takes an initiative test. Fail, and take a Strength 5 Ap 2 hit. But, who is hitting you in Close Combat enough to do a lot of wounds, and is low initiative? Only a few units, who are all high toughness. This isn't a great buy, but there's far worse. (4/10)
Regeneration: Costs six Hormagaunts: This is identical to It Will Not Die, but passes on a 4+, and it theoretically stacks with IWND. However, there is no way to get both. This isn't cheap, but it can help quite a bit because you're more likely than before to gain a wound back if you've only taken a few. At the end of its wounds, it can help an MC stumble along a bit longer. This is prone to focus firing however, as it only happens at the end of the turn. (7/10)
Bio-Artifacts: Can be bought by Tyrants, Tyranid Primes, Trygon Primes, and Tervigons.
The Maw-Claws of Thorax: Cost two Hormagaunts: They work identical to Rending Claws, but once you get your first kill up close they give you preferred enemy against that codex. At only slightly pricier than rending claws, on a Prime you can easily justify the slight increase. This also affects shooting, and he could then give it any unit he's a part of. Worth the increase if you were going to buy a Prime just claws. Less worth it on MCs, but if you are confident you'll get in CC it's not bad at all. The best part is the low cost, not being game-breaking. (7/10)
Miasma Cannon: Costs five Hormagaunts: It's a gun 36", 2+ poison AP 4 small blast. Can be fired as a template, same stats. For five points LESS buy a venom cannon, which is strength 9 and will almost always wound on a 2. And can hurt vehicles, cause Instant Death, and is cheaper. Dear god. If you want to throw this on a Tervigon or Prime in a support role it could possibly kill things, but... really? (3/10)
Norn Crown: Costs eight Hormagaunts: It gives +6" to Synapse, like Dominion. Only problem is, Dominion is almost free and this isn't. No, not free at all. This is roughly 4 times what would be an appropriate points cost. Never buy, ever, unless being bribed or making a bet like "I can win with the worst list possible". (1/10)
Ymgarl Factor: Costs eight Hormagaunts: This is supposed to replace my beloved Ymgarls? Ew. Every assault phase, choose +1 to one of the following. Attacks, Strength, Armor Save. This is the only way to get a 2+ save on a non Tyrannofex, but is only assault. This is just way too costly as most wounds will be taken in shooting when this does nothing at all. Maybe fun for a Trygon Prime? (5/10)
The Reaper of Obliterax: Costs nine Hormagaunts: This is a special Lashwhip and Bonesword, with more than double the cost. In exchange you get +1 strength and Shred. Or, buy a Lashwhip and bonesword, and Toxin Sacks, which will pretty much be the same for cheaper. This is only even a fathomable choice for Trygon Primes who can't buy the normal LW/BS. Even then, way too costly in my mind, but the initiative boost is kinda nice. (3/10)
Codex Impressions: Powers are pretty decent, workable to be sure. We have many bad options and a few good ones, I anticipate this books problem will be in variety not power. Stick to the right things and you can be fine, but there will be many similar Nids armies. And, Synapse is the number one rule. No one can be alone, they MUST be babysat. Out of LoS termagants on an objective used to be great, but now? No chance. Synapse synapse synapse, or die. Lots of things are now focused on expanding that synapse range, with a theoretical 30" synapse range. 18" is more likely though, as you'd need to roll the right warlord trait (unlikely) and but the Norn Crown (even less likely). It's sad that our armies unique rule isn't a benefit, but the threat of punishment. But hey, forging narratives means Humans killing leader-bugs. That's just science (fiction). What are you thoughts and impressions?