This is most problematic against Imps, which drain all mana, as the hero casts a spell like Skin or something else unnecessary, against Furies (in general, against all high-initiative creatures) it's also difficult to play, as they manage to move and if you stand opposite them, you have to restart the game. In my opinion, it's better to give a neutral bonus (+ to movement, or something like that)

Added after 3 minutes
I play as Dungeon, and it's more beneficial for me to cast a spell like Slow or Vampiric Touch than a random spell.