I'm not convinced Black Kyurem is broken in OU, even though I think the meta will be better off without it. The damage upgrade from Haxorus isn't that significant in terms of KO's, especially now Haxorus got Superpower and Aqua Tail. Also, it isn't hard to revenge when it locks itself into Outrage.
EDIT: I really can't see W Kyurem staying, or even being tested. That Draco Meteor backed up by Fire and either Fighting or Ground coverage to hit Jirachi / Heatran / Tyranitar etc...literally nothing can switch in safely, and most special walls risk being easily 2HKO'd if they mispredict (out of the very few which aren't destroyed by DM). Unlike Victini and Darmanitan, it also gets a lot of opportunities to just spam attacks, because its typing flat out beats Water / Grass / Electrics together with it's nice bulk.