There are TONS of bugs with spells not working properly all the time and a weird bug where sometimes the spell bar doesn't function at all, and all you can do it quit or die. And yes, Lil' Ragnaros and the Pandaren Monk are overpowered; if you get a match with those pets, just leave the battle. Really, it's not that hard. You don't lose arena points, just quit.
With all that said, pet battles are still a really fun and enjoyable aspect to the game.