Why do few small planes have reflexed flaps?
I'd guess that it's because gliders are optimised for low speed in rising air and their aerofoils have lots of camber. Reflexed flaps allow them to reduce the camber for high speed flight, but the resultant aerofoil isn't as good as a proper low-camber, high speed aerofoil.
RC power models generally have plenty of power to spare and either don't care about drag or it only matters at high speeds. Their aerofoils are generally low (or zero) camber already so they don't need reflex flaps to achieve it.