How is the functionality of EF on the EOS R?
EF lenses are EOS system lenses.
RF lenses are EOS system lenses.
The RF communication protocol is built upon the same EOS protocol that EF cameras and lenses use. It's the equivalent of "USB3.1" compared to "USB2".
RF cameras and lenses have certain additional/expanded capabilities in terms of lens/body communication that EF cameras and lenses do not. But there is nothing that an EF lens can do on an EF body that it can not do on an RF body.
When using an EF lens on an RF camera via one of several EF-RF adapters from Canon, the EF lenses lose
none of the functionality they have when used on EF bodies. NONE. On the other hand, they may not have certain functionality that RF lenses have on RF cameras. But that is functionality the EF system never had, so EF lenses do not lose anything when used on RF bodies.
Nothing.Nada.Zilch.Nought. Zip.Zero.