I would say the spell Charm Person is the one that will be the most difficult for a new DM to plan for, roleplay, and keep balanced.
I think it counts as "broken" that PCs will die unexpectedly, without their players making any bad choices, in encounters you had no idea would be tough. It really strains a game when instead of running the adventure you want, you have to leave the dungeon to get new companions, everything stops while people make new characters, all the attachment they formed to their character concept is gone...