When you consider the fact that Dusclops has absolutely no offensive presence, only Rest as viable recovery, and doesn't pull its weight at all, you realize that it has no business being UU. Maaaybe BL2 if we're being generous. The issue with Dusknoir is that it doesn't have a good STAB because it is a physical ghost, and doesn't have Swords Dance to do damage with its punches under Trick Room. Once people realize how mediocre Dusclops is, and once GF buffs Dusknoir, then it will probably the same tier of its prevo.