I think we are seeing that some of the native RF primes cannot be beat by their EF counterparts, but not because of the adapter.
From personal experience, the EF 16-35mm f/4L IS and the EF 35mm f/1.4L II are just as sharp on the R. So is the Sigma 180mm f/2.8 IS Macro.
The ef 70-200mm f/2.8L IS II is a little different story. It seems I'm not alone in having some trouble with the balance and steadiness on the R when held vertically. First of all, the adapter adds length, and, secondly, the R is smaller and lighter than the dSLR's I'm used to. Maybe I can improve this with practice, but for now, I'm avoiding the combo. It's possible there are other ergonomic aspects involved too, such as the shape of the grip and where the AF area of the touch-and-drag is set.
So, right now, my conclusion is that if a lens is short and light, the adapter is not going to be a factor in reducing sharpness. Or, as in the case of the Sigma 180mm, if it is so long and heavy it rarely gets used without a tripod, the adapter does not seem to cause problems. Lenses in between will be ok for some photographers, less so for others.