ive had carbides in my diffs for over a year and they're still smoother after a rebuild than any new steel balled diff
plus you can do the diffs right up without fear of putting a flat on the balls. i want to try ceramics because they're sposed to be a 10th the weight of tungsten and last forever. as for bearings id put ceramics in the drive train but yokomos tend to knacker their hub bearings in the first accident so i use flushed out £1 throwaway steel ones on the extremities and just change then often!