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."

[1] https://www.hambini.com/blog/post/ceramic-bearings-vs-steel-...

Much lower wear and slightly less friction. But they cost roughly 10x as much going from a quality steel bearing. They are marketed as being faster but it's not noticeable in real world conditions. They might be worth it in some non-cycling applications (high precision motors or something). They also break if abused. Steel bearings might deform the balls or dent the race, but they will still turn.

However, as long as there are people with $250 to spend someone will make a $250 set of bearings- don't forget to lube them with snake oil for the fastest results.