Personally, I think Kyurem EX works great in 6 Corners. Especially if you're in Seniors and people are still playing Reshiphlosion. Besides the fact that your opponent has to take two prize cards when they KO it, Kyurem EX has many downfalls. The below are just a few of them:
A Weakness to Cobalion: While CaKE is not the BDIF, nor the most popular deck in the format, a weakness to Cobalion could still be troubling if your opponent managed to surprise you with CaKE. Not just that, but Durant has been rising in popularity lately, and quite a few Durant builds run Cobalion.
Three Retreat Cost: A huge retreat cost could be a real pain here. This card's attacks will rarely OHKO your opponent's Pokemon and because of this fact and the huge retreat cost of this card, it will just sit on your bench as Catcher bait for your opponent.
It's 'best' attack is terrible: 120 damage might look good, but for 4 energy it isn't. Just look at Reshiram and Zekrom. They both do 120 damage for three energy, and with side effects that are no where near as terrible! So what is Kyurem's side effect? It can't use the attack during your next turn. Lame!