I wish I'd see one in the game... Hard to tell, since many campaigns are nearly decided before you get to have any Geisha, or even have the AI recruit any.

I doubt that they should not be that bad for the human player, since every other agent in the game has a mechanism for getting rid of enemy agents (Metsukes apprehend, Monks convert and Ninjas assassinate). At least I tend to make sure my ninja and monks know their business, my Metsuke tend to just tax the hell out of my towns.