I didn't had in mind fiber reinforced polymer bearings, that are explicietly made as a bearing, but of course that's interesting in this context too.I thought more of parts like plastic housings that are, by the way, bearings too. Worst case: just a hole drilled in, acting as a bearing for a spinning steel shaft.If the hole is already molded in, it's possible not as bad because the fibers are oriented more or less flat on the surface.But even then I can't imagine it's any good to let a steel shaft run in direct contact with fibers harder than the shaft.Possible the fiber reinforced polymer bearings have a coat covering the fibers and acting as a bearing surface ? But that's just a guess.In general the fiber reinforcement also has advantages as you told, of course.For example, on the fiber reinforced piston, I reamed out the hole to the next size of hardened pin I could buy.Now this pin sits tight inside the plastic material and is pivoting in the pump lever boring only. At this point the additional hardness of the plastic willhelp that the pin don't get loose (so I hope). And that's the pistons weakest point, I guess, where the piston material has to carry the biggest force one a very small area.In normal plastic material this pin would come loose again much faster, I guess. - so far -