Hambini has a good article on this[1]. The basic problem (apart from cost) is that only the ball bearing itself is ceramic; the inner and outer races are made of steel and the ball wears a groove into them which increases friction over time. Near the bottom of the article there's a 'power consumption vs kilometres' chart which illustrates this nicely.
"Hybrid ceramic bearings are the equivalent of trying to run a locomotive on an asphalt road - the hardness differential causes the road (raceway) to become damaged."