It requires a complex, expensive, sensitive, tuned control system with more parts and a decent amount of cpu power. Also, they're subject to axial translation.

It requires a complex, expensive, sensitive, tuned control system with more parts and a decent amount of cpu power. Also, they're subject to axial translation. And it saves friction, not energy, so the benefit is maintenance, si it's only ptavtical in some apps. Still as the systems get more predictable they're finding their way into more applications...