Planeswalkers

just so powerful against every color

the only option to survive against this commander is to have a bunch of your own colorless cards

fun to play as

seems a bit too strong to play against

While my general response to Ugin on an emotional level is "ick!no!" I don't think it is broken enough to be banned. There are oppressive commanders in Brawl and Ugin definitely is one but it is entirely beatable. The fact that you are seeing it at all should mean your deck has its weaponry to deal with him.