Mobility: We have, Stormraven's which can deploy our troops reliably over 12" or not so reliably moving Flat Out, and we have Interceptors and Dreadknights which can be Jump Infantry and can Shunt once per game. All of these units are good albeit, but they're all also VERY expensive (to the point where I can typically only field about one unit of Interceptors and one Dreadknight in my 1750 pts lists,) AND they have their own hosts of weaknesses as well. What do Tyranids have? Gargoyles, Harpies, Shrikes, and Flyrants. Do any of these have the sheer 30" movement of the (once per game) Shunt? No. But are these all fast as hell and undeniably dangerous? Of course. If that wasn't enough then lets not forget that Hormagaunts have super fleet, and most of their stuff can either Outflank or Deepstrike into position. Unsuspecting players can very easily get entrapped by a clever Tyranid player, and the game is over before they even know it.
Gargoyles cannot hurt vehicles. Tank shock, incinerate, dead.
Hormagaunts cannot hurt vehicles. Tank shock, incinerate, dead.
Harpies are never fielded because they're both too fragile and too expensive for what is effectively equivalent to a single BS 3 lascannon that is more likely to glance than do actual damage. AP- is a real problem.
Shrikes are never fielded because they're both too fragile (stormbolters and incinerators deny them armour saves entirely so they just die) and too expensive. Like warriors, krak missiles kill them outright. GKs don't have krak missiles, but they do have psyflemen, which are just as bad. Worse, even, because of their accuracy. Since 50% or more of players play Marines (including GKs), you won't see many Warriors and Shrikes on the field unless you're list tailoring against your opponent, because they can't be justified in an all-comer's list when half or more of all comers will just kill them outright. And the final nail in the coffin: they can't hurt vehicles.
Flyrants can possibly hurt vehicles, but they're also prohibitively expensive. If you buy guns for them -- and I grant that devourers on a flyrant can get side armour shots -- it's both yet MORE expensive (even without any upgrades a flyrant is A LOT more pricy than a stormraven) AND it's still not enough to do anything against AV 12, AND it's still difficult to actually damage vehicles because of AP -. And like I pointed out earlier, a single salvo of mindstrikes will just kill it outright anyway, so I have no idea how one can expect a flyant to assault a stormraven.
Armor: Is this to mean that we actually have vehicles with an Armor Value rating? If so that's not something I consider hugely impressive. Like other armies, they definitely have the weapons to damage our vehicles, and the case has been made time and time again that Monstrous Creatures have SERIOUS advantages over vehicles. If it's to mean literal armor value of infantry, I wouldn't put to much credit here either: we're only normal space marines in terms of toughness but have almost twice the premium. They have the numbers and the special rules/abilities to make it to our lines.
See what I said above already. Tyranids have a terrible time punching armour. There are only so many hive guard units they can take. Zoanthropes are regularly stuffed by psychic hoods (and they suffer from extremely short range anyway). Tyrannofexes are very good, but also prohibitively
So 'nids can either be as shooty as possible, which means pouring points into MCs and hive guard so they can have a slight chance of damaging mech armies, OR they can build for assault. They can't do both. If they build for assault -- be it swarmy or monstrous -- the mech army just tank shocks, flames, and puts up roadblock after roadblock to slow the advance down until the army is shot and assaulted dead. If they build for shooty, they simply can't match the points efficiency of any imperial army, GKs included. So GKs simply outshoot them through weight of dice.
As for infantry survivability, GKs only ever have to be in combat when they can dictate the terms. They can throw Rhinos and Razorbacks in the way and force 'nids to assault the vehicles, all the while flaming them to death while doing so. Games only last 5 to 7 turns, and 'nids can't get very far when they aren't allowed to move far and are forced to assault their way out of trouble.
Shooting: Grey Knights undeniably have an assemblage of the most elite shooting in the game, no one is denying that. But can we honestly say that we JUST beat them at shooting? Is this statement in any rigorous sense true? In terms of strength they can bring a fearsome amount of strength 5 weaponry to bear with Deathspitters and the like. Nevermind the sporadic str 6+ large blasts that can be thrown in their from time to time. Let's not forget about their famed Hive Guard (who have the same effective threat radii we prefer to operate in,) Zoanthropes, Tyranofexes, or even the bio-plasma their Carnifexes can bring to bear. This all only being to list a few.
Deathspitters are prohibitively expensive. And where would you put them? On Warriors? Like I noted above, who is foolish enough to field Warriors? Let alone in any quantity to cause problems for any Marines army, GKs included?
I already covered the limitations and extreme expense of Tyranid shooting. Some of the "solutions" posted on the hive mind website included using Venomthropes to provide cover. That's fine -- I use them myself sometimes -- but every Vthrope unit is one less unit of Hive Guard or Zoanthropes, too.
You make it sound like 'nids have options
, when they actually have restrictive choices
. Unlike Imperial armies, they can't have their cake and eat it, too. In Imperial armies, offensive and defensive capabilities are available in every force org slot. GKs can field mobility and/or assault and/or shooting (including anti-armour shooting) in every force org slot of the codex. Not so Tyranids! Anti-armour shooting is restricted to 3 Elite slots and a monstrous creature more expensive than a land raider. In general, if they choose increased protection in the form of Vthropes, their offense suffers. If they focus solely on offense, their defense suffers. If they want a horde, they can't shoot well. And so on and so forth. Imperial armies -- GKs included -- never have to make these kind of design tradeoffs.
Close Combat: Our most clear advantage here is against their monstrous creatures (which again, here and elsewhere it's been said that they should be avoiding our troops to a great degree for this reason,) but unless literally ALL of your strikes and GK's are toting around expensive Halberds, it's unlikely Genestealers aren't going to do some serious damage. Nevermind when their Warrior variants hit our lines with Lash Whips and Boneswords so that even with Halberds we'll be striking at a far more modest I3. Even with halberds I wouldn't suggest taking the full brunt of a Hormagaunt charge with their Toxin Sacs and Adrenal Glands...
The reality is that GKs are in control of when and where assaults will happen. Strikes and interceptors prevent advantageous deep strikes. GK mechanization tank shock and burn hordes off the table. GK mech also dictates where 'nids can move freely. Stormbolters thin anything not an MC down to manageable size very quickly. Psycannons (and other vehicular weaponry) rip monsters to shreds. Things like stormravens and land raiders deliver assault units reliably on target wherever the GK player wants them because the 'nid opponent is all but helpless to stop that, so how can they avoid being assaulted by init 6 force weapon death?
Psychics: Our best psychic defense honestly comes in the form of our dreadnoughts and requires us to be within 12" of them. The same is true for their SitW and their Synapse creatures, but for obvious reasons they have far more of them than we do dreadnoughts. In terms of actual psychic powers I personally don't consider any of their Zoanthropes powers, Paroxysm, Psychic Scream, or Catalyst to be minor threats to our army.
Making normal psychic tests on an effective LD of 6 is harder than passing psychic tests on 3 dice with LD of 9 or 10. That's just basic statistics. Zoanthropes don't have a far enough range to be a threat: it's not hard for GKs to shoot down (or even assault) 6 MEQ wounds. (And that's kindly assuming zoeys don't fail an important save when hit by a psybolted autocannon shell. If GKs can spam psyflemen shots on zoeys, they won't even last as long as 6 normal Marines would against that.)
Paroxysm is, I admit, truly awesome. WHEN you can get it off, and IF your tyrant lives long enough to get the shot off. A Flyrant has no protection, and so usually is dead before Paroxysm is in range. A walking Tyrant is too slow, usually, to survive to get within range, even with a full complement of tyrant guard. Again, still assuming the power will go off on a LD 6 test. And if you bought all those Tyrant Guard, what was sacrificed to pay all those points for them?
Not sure why Psychic Scream is considered valuable when one slot is ALWAYS going to go to Paroxysm and the other slot will pretty much ALWAYS go for leech essense, because you need the ability to get wounds back to have any chance of surviving long enough to do anything.
Catalyst can't help against any close combat attacks by GKs. It's only good against shooting, where it does help. But you're not gonna have more than 2 or 3 units with FNP at any one time -- and that's only if you buried a lot points into Tervigons. Tradeoffs, remember? If you built a hordey, tervigon-heavy army, you have no credible shooting phase. Suddenly the GKs can just destroy one tervigon per turn and blow the army up. If you only have 1 tervgon, well, who cares if one unit has FNP? I agree: it's a terrific power. But it's no panacea. It is one of the few things that makes 'nids even halfway playable these days. Take away Catalyst and I wouldn't ever play the army again because it'd be all but pointless.