The Uber tier is effectively a banlist for OU, however this does not mean it is not, or cannot be used. Pokémon in the Uber tier are deemed too powerful or centralizing to be used fairly among other Pokémon. It is a misconception that the Uber tier contains only legendary Pokémon. While it is true that a majority of Ubers are indeed legendary, not all Ubers are legendary, nor are all legendary Pokémon Ubers. While most Pokémon in the Uber tier are there due to high stats, Wobbuffet is often classed as an Uber because of its Shadow Tag Ability, along with Encore, its high HP stat, Counter, and Mirror Coat; this lets it trap any opponent Pokémon and lock them into any move, so that the player knows what their opponent will do next, which allows the player to either switch out Wobbuffet knowing what the opponent will do, or use Counter or Mirror Coat to damage the opponent. Wobbuffet's pre-evolution, Wynaut, is also often banned for similar reasons. Wobbuffet is not usually Uber in Generation V tier lists. Blaziken is often classed as an Uber because of its Speed Boost Hidden Ability, which when combined with Swords Dance, Blaziken becomes very powerful and faster than most opponents. Other non-legendary Pokémon such as Excadrill are considered Uber due to their movepools, high base stats, type, and Abilities. Conversely, Kyurem (in its normal and Black formes) is not considered Uber despite its very high base stat total and lack of a hindering Ability such as Slow Start, due to its Ice typing and comparatively small movepool limiting its options against other Pokémon. 5th Generation Ubers (With Speed Boost Ability ) from: Bulbapedia and Smogon
Wobuffet is OU. Blaze Blaziken is banned as well, which is really, really lame. They've yet to put an ability ban on it.
Yes and no. They'd be OK, but you'd probably do poorly unless you had a heck of a strategy and/or your opponent was pretty lame. If ubers are banned, you are expected to use mostly OU Pokémon. Of course if you can fit a UU in your team and it actually works better than a superior Pokémon, then of course you should...
oh. so people don't try to win with their favorites? like Elite 4 Karen once said, "Strong Pokémon. Weak Pokémon. That is only the selfish perception of people. Truly skilled trainers should try to win with their favorites. I like your style. You understand what's important. Go on–the Champion is waiting." o_O