ef lenses are compatible and work well. but not equal to RF lenses, eg any functionality dependent on that 12 pin communication interface/bandwidth or built-into lens gyrosensor drive for "enhanced IS", and new AF drives (eg thin-type nano usm/Linear EM drives) for more advanced AF functionality etc.Except, this is not what people who have used EF on the EOS R with an adapter actually say. They've said that it works like a native mount lens. And really, there's absolutely no technical reason that it can't when Canon is designing the body and adapter specifically so that they can be completely compatible.
RF lenses are designed for performance and functionality beyond EF lenses' legacy limitations.