Smoothness and roundness. Ceramic balls have a smoother finish than steel. That affects directly in the roundness with grades up to 25/1,000,000 of an inch of accuracy. This reduces vibrations and a uniform contact between ball and ball surface will improve lifespan of the bearing.
High rigidity. The compressive strength of ceramic materials is about 5 to 7 times that of steel.
Corrosion resistance. Bottom bracket bearings are very exposed parts from moisture and dirt. The chemically nature of ceramic balls bearings makes them much more resistant to corrosion than steel.