His attacks are extremely predictable, and slow. He also gets destroyed easily when someone is very close to him. IMO he is a waste of space in the game. I don't see how anyone could possibly be good with him. If they were aiming for comic relief they kind of missed....
Besides, I don't know if it's just me and maybe I just suck, but Kefka's Forsaken Null attack is annoying as hell. Pretty damn cheap. It's true that he has almost no close range attacks, but who needs them when you've got an attack like Forsaken Null?