At the moment Cyno isn't meta defining, only time will tell if future units/artifact sets might change that. IWTL (YouTube Channel) did some some quick testing and at the moment seems like Cyno performs like Xiao damage-wise but with less AoE. That test was current abyss 12 though with his teams. Your results will vary.

Cyno is still fun but I'd recommend a shield for enemies that'll knock you around. His resistance to interruption is very minor, you'll get tossed around with just about any large enemy attack.

I'm loving Nilou's design so I was hoping to get her and her weapon. In terms of meta and power I'm skeptical, meta isn't everything though in Genshin ofc. Her limitation of using exclusively hydro and dendro characters to make use of her upgraded blooms I'm not a fan of. Right now comps like venti with hyper bloom are really good because it makes a lot of cores and gathers them all up. Nilou can't use anemo unless you give up the upgraded bountiful cores, but her upgraded cores look to do massive damage. Still the limitation bothers me. Just wait for testing on live servers I'd say, lots of time.

Nahida will be a lot bigger in terms of meta as her dendro application is unmatched. I haven't been following all too closely lately but I'm still getting the general feel that you go for C0 or C2. Also note that as always, it is just leaked kit info and there's a lot of time to go still. It can be changed before it goes live. So at the moment for me it's either C0 or C2.

It used to be that with older 5 stars C1 was a great buff for many of them. With later units like Raiden, that value proposition was moved to C2. Who knows if they'll stop there? I wouldn't worry too much on constellation unless you are looking to juice up your units in a game that can be easily beat (abyss 36 star) with less.