Freeze is absolutely busted. If you get it and enough other free cards, you can loop between those cards and stall the enemies indefinitely. You can even have a one energy cost card in there and still always be able to continue the loop.
But then you don't progress in the game. The goal of the game is to get to a high level, pausing the game doesn't make you win.
There are definitely broken combos though, and freeze is way too powerful. I don't think there should be any free cards in this game, being able to cycle cards for free is way too powerful. Most deckbuilding games have other ways to limit this like being turn-based, but even then they almost never make cards free.