>>22699318As for how viable I think they'll be, I'd imagine Hoopa-U to be A- in OU at least, though it could end up A+. Hoopa-C will probably be C rank, since it can fuck up Chansey and stall a little better because of its typing. Maybe C- because of that nasty defense and typing.
Something I haven't really seen mentioned is how Hoopa-C can run a physical coverage move like Ice Punch or Gunk Shot to deal with specific UU mons, which could be useful in situations if they are a bit niche.