so, their psychic phase got worse.
it's not significantly worse, you're likely to get catalyst off just as frequently as you used to by dumping dice into it, and it's the only spell that really matters.
Other armies psychic phase got better,
arguable, the casting %s are down across the board and the perils %s are up (which are affected by shadows). some of the powers are better but largely you're seeing the same nasty stuff as before with lower cast %s. invis being the exception.
Shadows now does less.
it was only important before because it made psychic tests non-auto-cast, they're now more like that by default. i'd like an faq to make it do more, but i'm not holding my breath
Nids already weren't doing well.
they were competitive with skyblight, and now that's a standard part of the game.
Nids still can't ally with any except in 'come the apoc'
pure upside. i can now ally where i couldn't (including self-ally), don't see an issue here.
Taking a Jink save now means that you can only shoot snapshots.
that was already like that, and the jink went from 5+ to 4+, again pure upside.
Vector strikes now do less to ground targets.
gotta give you this one, weaker vector hurts.
One of their powers got nerfed, as did one of their unique items.
if you're referring to warp blast, i agree it's worse, but it doesn't really matter that much, not a lynchpin for the army and it technically did get a minor buff as well since my flyrants can now cast it and still shoot all their guns. all their unique items were already terrible so that's moot anyway.
realistically the army got a few huge buffs: the grounding changes are massive, formations being in is massive, lords of war being in (assuming they stick in competitive) is massive especially since the nids gargantuans never had d to begin with so the d nerf is pure upside. combining the grounding buff with a t8 flying gargantuan makes for a really hard model to kill (granted, 735 is pricey).
all that combined with the fact that the deathstars are generally harder to build or less reliable (invis aside) and the fact that the fmc changes probably hurt daemon flying circus more than nids since they were more reliant on assault than nids, the likely meta shift will probably be favorable for nids (again, same caveat as above about the number of 7th ed games i've played
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while it's too early to say nids are top tier (realistically, they probably aren't), i think it's also too early to say they're trashcan.