There good no doubt. But there are a lot of good models. Belle's provide some of the best control in the game, in general ressers are very strong positional control faction. Its hard to write a resser list without a belle in it, or without a plan to make at least one they are very good. But they themselves are not game winning, they set up your other pieces for success and if you bring a friend pounce is just nasty. Not to mention the trigger to get rid of cards on lure.
Overpowered no, but limited to 3 absolutely.