The main problems that will occur with Queen Mab are:
- A full team mana drain for 14 mana cost would be a lot. On top of that, hers is a mana burn. Her ability is Psion's on steroids for 1 mana cost LESS. Only difference is she does physical instead of true damage.
- Her 4x/5x freeze is better than Bone Dragons's freeze on skulls. Bone Dragon and chilling touch can only freeze the first enemy. Queen Mab can freeze the entire other team
- Her arcane trait will raise her to ridiculously high damage output with the right quick cast team. She is also fey, so +2 magic for 4x fey. We don't know yet how mana burn scales off of magic, but higher is definitely more dangerous.