If you just swap them in, the main advantage is improved durability especially at high rpm.  Ceramic bearings can survive in less oil, so if you can limit the oil to what they need ( mist lubrication is ideal ) then they can save a little drag.

SKF showed that just swapping one ceramic ball into a regular bearing would greatly extend the life.  Theory is that the super hard ball irons out imperfections in the hardened steel races.

OFTEN NOT!  But once bought, you're stuck with them...
The main problem (as I see it) is the internal clearance. If you have a supplier that guarantees the internal clearance (c-3, c-4 or whatever), they could work well. With many suppliers (or 're-sellers'), it's hit & miss. I bought ~ 8 Boca's before I found one that actually ran well in my Shuriken. My attempt to fit them to my buddies pattern engine was a complete failure. Hardly an efficient way to spend one's money...

p.s. I was in Moscow years ago with Sergei Kostin, rebuilding one of his F2A engines. I noted Sergei removing ceramic balls & replacing them with steel ones. I asked why. Sergei claimed (at the time) that the only difference was the amount of 'noise' the engine produced... HMmm! As far as I know, the top F2A Speed Engines are not yet using ceramic bearings even though a lot of material out there indicates they ought to be better...

p.p.s. So far attempts to change to ceramic bearings in a Moki .15 (Clown) motor, K&B TQR motor, Nelson .40 Pylon motor, & a 1.20 Pattern motor all seem to have failed.

p.p.p.s  Bill Hughes has reported that one of his F2A profi .15's was ruined when his ceramic balls "turned to dust" during a run...OUCH!