Well, the biggest con is they really aren't all that great.
Ribbons are very fast, but flexible. So good at frequencies so high your dog can't even hear them, because those don't call for moving much volume. Go even a little lower though into the midrange and now the air they have to move is too much for their soft design.
Ribbons also have hard to drive low impedance, which calls for a transformer, which then the transformer becomes a factor.
I honestly never understood why anyone would use them, especially since I never heard a speaker with a ribbon that sounded any good to me. Actually now that I think about it one of the worst systems I ever heard had a pair of Carver's Amazing Speakers which feature a great big long ribbon that I think tried to get around the physics of low frequency by making the ribbon 5 feet long. Or something like that. Plenty of other reasons that system was so awful, can't put it all on the ribbon. But still....
