Believe me, they do. I had a huge difference between left and right leg (always causing backpain during long rides) - after 3 months of powercranking, the strength difference and back pain were completely gone. I don't need a scientific study to back this claim - I am just happy everything is solved. If people think that is anecdotal and worthless, fair enough :-)
I personally know 1 person who very successfully used powercranks to recover from a severe hipfracture (resulting in a huge imbalance). He is a pure cyclist and got his strength back very quickly as soon as he started riding independent cranks.
There is much more examples but some people just don't want to be convinced about anything positive related to them.

Sam
samgyde.com