I still feel like Y’Shaarj requires a lot to even pull off (you have to corrupt the cards then play them in the first place) and by the time you play him, midrange and aggro decks should’ve already won, and control decks should have the answers.
There is no such thing as a broken card after turn 10. Midrange and Aggro decks are supposed to have lost by then. Control decks might still survive because they’re fully capable of clearing a Y’shaarj board. And because control is so capable, a well-planned Y’shaarj swing is perfect to challenge them.
It has huge deck building restrictions. Corrupt cards have to be in the deck! Those cards are only tied to this expansion as well. Additionally those cards have to be corrupted which affects the other cards that have to be in the deck, typically of a higher mana cost. Then those cards have to be played before the corrupt cards.