Overall, the idea of Cress in UU doesn't really impress. It's defensive stats are impressive, but considering UU's already got pokemon like Registeel and Chansey, Cress's defensiveness statwise hardly seems overwhelming. Not to mention Uxie has been in UU since the get go, so strategically speaking it's not bringing a whole lot of functional diversification to the meta..
Beyond just stats, the biggest problem cress had in OU to begin with was that it just doesn't really do anything. It can thunderwave, but there are tons of pokemon in every tier that can do the same. It can set up screens, but it's not even the most efficient at that, Uxie getting better speed, Momento or U-Turn, not to mention Stealth Rock. Even Alakazam seems a more interesting screener with Taunt and it's awesome speed.
The only reason you'd even use Cress in OU is because of Salamence. It's typing isn't weak to ground, fire or dragon and its defensive stats are just enough to handle Salamence on both sides, being fairly balanced. Obviously, Salamence's presence alone did not justify Cress being OU (especially when Cress is raped by Mence's buddies Ttar and Scizor). I wonder if Cress will be like P-2 in UU-- without Gyarados around, P-2 just doesn't seem that impressive, and I think Cress might find itself in the same situation in a tier with no Salamence.
On the plus side, it does get moon light in a tier with no ttar. :O And with no Scizor (or Honchkrow now), Cress will be a tough cookie to crack. Only problem being-- even if it's super hard to kill, again it can't really do anything threatening to even most UU pokes. :/ You might as well spend the day watching two metapods fight.
And again with Registeel and Chansey around, P-Z might find itself in a rather similar situation to where it was in OU. :P