Their upgrade certainly pales to the original MP dragons but I'll take any buff we get. They all got better at doing what they already do with a stat bump, awakenings, and in some cases leaderskill buffs.
You Yu and Xiu Min are the big winners, Xiang Mei and Plum are better than they were but not "must haves". That said I'll be making all of them. If Xiang Mei and Plum's LS got buffed to include an additional typing I think they'd see real use again. Maybe JP can complain for it.
I feel like they kinda might play better than they look. Maybe not a lot better, but somewhat. In particular Xiu Min seems kinda reasonable. Still very orb hungry, but pairs well with either ameno or Yog; With yog, matching two light combos (and nothing else), is a useable 45x multiplier with a shield. When you go whole hog you end up with 315x. I just wish his active was better. Ameno really forces you to make a row plus for any damage with this set up, but the 25% shield will be useful and you won't miss damage on burst turns.
As for Yo You, I've been burned by 5o1e before, making it seem less awesome than I'd like. two 7c is nothing to sneeze at though and he can really use some of blue's TPA subs pretty well.
As for Xiang Mei, she still seems like she needs a system, and her damage is pretty low considering you you need six hearts to make it work. I also don't know of any healers off the top of my head that have really helped her team since her initial release. I think she is still possibly viable, if you want a system, just due to the fact that you will probably have solid HP and RCV simply due to typing.
Xin Hua
might work, but it depends on how many hearts she needs to pull in full damage. If she stays at 10 heart orbs, there will be no helping it; it is just unreasonable. She also doesn't have HP, which is quickly becoming more of an issue as we go along. I will say, that she is interesting if she has the full multiplier at 9 orbs or less, as, with a system, that means that the super FUA would give 2x to the majority of the team, which could be pretty crazy. Probably not worth the trouble, but rather cool nonetheless.