At this point, "UU at best unless it's perfect across the board" is a good rule of thumb. Nature of power creep. Every new gen, there's new mons. Unless they all suck(which they won't), the most busted S-tier of those mons will end up in OU, which means the weakest part of OU drops. Go on long enough, and even gold standard mons won't be OU. Think about current UU.
Mence, RotomW, Keldeo, Excadrill...if the standard for a mon is that it has to be better than those to even think about OU, then there's not much that can do it.
Fun fact, less than half the pseudolegends are OU. Dragonite, Tyranitar, Garchomp, Dragapult make it. Metagross(RU), Salamence(UU), Hydreigon(UU), Goodra(NUBL), Kommo-o(UUBL) do not.