Mavic had ceramic coated ("CD") rims at one time, on the brake track, and they worked reasonably well but at some point the ceramic would chip off
on a limited area. Had that happen on two sets of rims, surprisingly it did not grab while braking over the chipped off areas which were 10-14 mm in
size. Mavic discontinued these about 5-7 yrs ago. The price premium over non-CD rims was not much.

On another note it would be interesting to know what brake pads the TdF riders were using on the rainy downhill 40+ mph descents with carbon rims.
Seemed like they never had any trouble braking even with pouring rain.