Well yeah most decks don't use him because they don't have him 😂
I wouldn't agree with Managamar that he utterly breaks the game. In fact it seems balanced as far as Hearthstone mechanics go. It's just really, really stupid to deal with. And it increases the luck of the game tremendously in a game which is already hilariously luck based to the point of insanity.
If you get the perfect Cthun setup, it's an auto win, if Cthun is the last card in your deck, it's an auto lose. Or at least it's going to be a hell of a lot harder to win.
It's one thing to be a card game, but there's no competitive card game in the world that's even remotely as luck based as Hearthstone. It's on a whole other level. Magic the Gathering seems tame in comparison. I brought up Duelyst which is extremely similar to HS in many ways but has no luck based cards at all and STILL has plenty of luck involved just in the draw.
Hearthstone just reeks of bad design. I could show you example after endless example of competitive game where one contender outplayed his opponent all game and then lost to something insane like a Doomsayer out of a Shredder or a Dr Balance out of an Unstable Portal or any other combination of nonsense the game can randomly throw at you.
Cthun only adds to this problem, if you can even call it that at this point. It also has the added benefit of being much easier to use than to counter. Because a 20/20 Avenging Wrath is pretty good in every situation, and the cards that build up to it are just as strong or stronger than regular cards. Average players aren't going to know how to handle this because it r@pes midrange. You either have to kill them quickly with an aggro deck or out control them with tons of removal and even more lategame legendaries.