Seiton definitely needs a rework of some kind despite having the same weakness of Meteodrive. Not only it's unfun for enemies, but it's not rare to get fucked by server ticks and missing a kill on someone who clearly was below 50%. Attentive players can walk in the middle of their team if they know they're going to get screwed by a NIN LB and the party can capitalize on the gapclose and animation lock. But realistically with a 2.5 GCD it's not something that happens consistently and NIN has tools to get out of that situation anyway. I think something fun would be reworking Seiton into giving you 2 or 3 charges of some buff that lets you use Assassinate outside of Shukuchi. Very strong but still wouldn't be as busted as Seiton is currently, and it'd also force NINs to get more in melee range. Other than that NIN is perfectly fine please stop needlessly nerfing everything but the chains Jesus Christ.
I think Meteodrive is okay simply because MNK doesn't have much to offer outside of that. The strong aoe burst requires constant GCD uptime to use frequently compared to something like DRG, point pressure can be very clunky to proc, LB animation locks you so you're incredibly vulnerable during it, and you can still survive it as a squishy if the MNK stuns before Enlightenment (which they do all the time in Diamond and lower and a fair bit in Crystal I've noticed). It's just that most likely you'll get cleaned up right after, though. 75s Meteodrive feels fine I think. Though I wouldn't mind if they nerfed it further or just reworked it and buffed RoE in some way, so instead of being a one-shot meme job it becomes a countertank. I feel like that would be much more fun to play.
Spite is fine.
Zantetsuken definitely needs to go though. Absolutely needs a rework. Even in a perfect hypothetical scenario where the enemies NEVER EVER hit you while Chiten is up, that in itself is absolutely insane value. A 5s invulnerability every 30 seconds. That's insane uptime. Not only that, but like you said aoe heavy jobs just have to sit it out and spam their 111 filler button until Chiten goes off. And realistically that never happens because someone is just bound to slip because no one can play perfectly all the time, so it's just an incredibly high chance of getting one or more kills. Shields can be wonky because due to server ticks you have to apply them preventively most of the time and not actively if you see Kuzushi on you, since the SAM LB snapshots immediately. Rework Zantetsuken, turn Chiten into an actual mitigation button.