I'm not a ballistician, just an engineer...
The real issue is not the breaking of the plate, but the invisible microfractures that can develop on the inside from abuse. The plate will look and feel fine, but will have significantly reduced ballistic protection. This is why a poster talked about X-rays.
Metal plates can possibly have the same problems due to heat treating and other defects. However, a bad lattice structure in steel tends to be because of the manufacturing process and not because it was dropped.
Ceramic plates might be "better" because the military uses them, but they have the money to buy new ones periodically and the equipment to check if the plates are still good. Steel should have the advantage that if it was made correctly, it should be good up until the time it takes a bullet.
YMMV, but I'd love to hear from someone who has some real applicable materials experience in this area.