Is mence really this unstoppable force, or is it a matter of a few people with reputation and influence not liking him? Even with Latias, it was found to be OU how many times? What changed all the sudden?
I'm a complete newb, only recently having stepped into the world of competitive battling, so my opinion might not mean much but I find him pretty predictable.
He almost always runs one of two sets; DD or mixmence. In addition to that both sets almost always run the exact same moves and EVs etc. Sure he's versitile and able to sweep teams in the hands of a skilled player or in the face of an unprepared team, but can't that be said for a lot of different Pokémon?
I can usually tell what he's going to do based on what he switches in on, and moreover he usually either fires a draco or prepares to set up a DD. Unless I have good reason to believe otherwise I assume I'll be switching into the more imminent threat of a mixed set and act accordingly.
I know that just because I don't have a big problem with him doesn't make him not uber, but anyone that allows him to come in safely without a good answer can expect to get slaughtered and that should be a no brainier, and isn't that the way it is with a lot of Pokémon?
Also I think people have to remember that he holds an important place in the meta, and in his absence a lot of potent threats no longer have to fear him. That might not be a good thing in all cases.