I do not thing Flame Cannon is broken as long as its numbers are being restricted. It alters the tactics you need to do. If you want to bring it forward, you're moving slowly. If you team it up with cannons, you basically restrict them to being cannons protectors against frontal charge.
I played once against the young dwarfs using their full artillery points for the flame cannons, and in great concentration, it really was devastating. As long as they're restricted, I do not see a problem with them.
However, that is, I believe, for a different topic.