There are two cards that simply break the fun completely and they are WIDELY abused.
Reverse Time and Black Hole.
I don't think I have to explain why, just go in arena.
Every other deck is 6 reverse / 6 black holes.
While not overpowered if used once, they just break the game completely when spammed because you basically can't play the game itself.
Everyone just quit the game when spammed with black holes and reverse time consecutively, as the game is basically over.
I can stand being turn-locked one, two turns, but not the whole game, as I don't even get the chance to play.
What's the point of playing if you can't play while playing?
Solution 1: Limit them to 1 of each per deck. 2 would still allow game breaking as one can get all 4 of them still.
Solution 2: Tone down Black hole to steal a lot less quanta and make reverse cost 5 time / 4 time upgraded.