The wheels inside this engine also roll, not slide. There are 2 variations of this motor, one with caraiage, and one without.
As far as the sides of the rotating assembly sliding on the inside of the housing, it should be similar to a wankel in terms of friction loss.
The problem I can figure out is how the wheels in the cariage keep from building up any carbon deposits. Even if it runs really lean, eventually there will be some carbon build up on the wheels, or in the bearings. I think this would be especialy true if it had to have oill injected or pre mixed like a 2 cycle or bertos rx-7.