sooo lets see, in summary...
DDmence is not the problem, since its almost the same as ddnite (which i belive does a better work than mence) (and even gyarados is bulkier than mence)
so this leaves the mixmences as suspects?, now, whats the problem with them:
-unpredictible? as any pokemon in the game, any pokemon that could be holding a scarf or band makes it unpredictible, something with an unusual build could be dangerus. thats what makes the game interesting and more important, challenging. personally i didnt had problems beating him, an average scenario, he kills something, with draco or outrage, then i send metagross and its over.
- hits hard from both sides? yes, but after 1 attack (that could posibly go bad) mence its an open book, draco meteor, no more spatks coming from him, just outrage earthquake or roost. outrage? he is traped on it. if he flees? he will take another chunk of hp from the SR.
-decent bulk? yes, he has intimidate, the same as gyarados, wich is even more bulkier than him, not the same speed, but in DD builds works better.
95/80/80 is pretty decent, but salamance has 3 weaknesess, Ice, rock and dragon. (wich are very common attacking types. ) plus, almost every mixmence has a naive nature, (some hasty).
he is strong, yes, he is an uber? not so sure.
in overall, he does not even reach the reasons why garchomp and latias went to ubers.
pletora of pokemons are as strong as him, thats why they are in ou, because they are better THAN other pokemons AT SOMETHING.
like scizor revenge killing, infernape at sweeping, tyranitar at being tyranitar and bla bla bla.