They're unreliable oil burners. They don't make enough power compared to the fuel they need to consume. They can be fast with forced induction, but so can any other engine, and nearly every other engine will do it more reliably.

They've got some cool ideas about them. The idea of being able to pick up an entire engine assembly with your bare hands minus accessories is kind of cool. The idea of making power via a purely rotating assembly from start to finish is also kind of cool. But 'kind of cool' isn't enough. In the real world, they just don't work as well as they should to be competitive with already existent solutions. Why do people like these things?