Disclaimer: Use of this information is at your own risk, and "All" is more likelihood than fact as I have only tested a small percentage of the available lenses.
In a recent article, I answered the question, "Can Canon RF Extenders be used with the EF-EOS R Mount Adapter?"
That answer was "No."
The Canon Mount Adapter EF-EOS R (basic and control ring versions) has no optics. It simply moves the EF mount forward, offsetting the short back focus design of the RF mount, and provides the communication connections required for the lens to function as native. Hoped for was that the open space in the Canon Mount Adapter EF-EOS R would permit an RF extender to fit into the back of it, ideally making all EF and EF-S lenses extender compatible. Unfortunately, the Canon RF extenders do not fit into the Canon mount adapter.
However, the baffle physically preventing this installation appears to be plastic, and plastic is easily modifiable. I couldn't spare a Canon adapter for this mod (they are unavailable as I write this), but after studying the Vello Auto Lens Adapter for Canon EF/EF-S Lens to Canon RF-Mount Camera product images, I suspected the relatively large inside diameter of this adapter could more easily be modified to permit fitting of the Canon RF 1.4x Extender and Canon RF 2x Extender. B&H was up for this challenge and sent me an adapter to potentially destroy.
Not surprising was that the RF extenders do not fit into the back of the Vello adapter out of the box. Thus, some modification was necessary. Basically, the plastic on the raised interior section housing the communication pins needed to be removed down almost to the wires. To protect the adapter and to keep it clean, I gaffer taped everything I didn't want to remove — most of the adapter. Next, a rotary tool (I substituted a drill) with a small abrasive sanding cylinder was used to remove the plastic. A rounded file could also be used.
After removal of enough plastic for the extenders to fit (this took two tries for my first attempt), the adapter was cleaned and mounted. Being able to mount the extender behind the adapter was the first test to pass. Having the extender function properly in conjunction with EF lenses was the bigger question in my mind. Fortunately, the modified adapter passed that test also.
How good is the image quality? We haven't lab tested the modified adapter and more testing needs to be done, but this combo seems to deliver very good quality images with a high quality lens in front of it. The Canon EF 24-70mm f/2.8L II USM Lens seemed to perform especially well with the image periphery perhaps becoming slightly softer. Here is a 100% crop from a Canon EOS R5 image processed from RAW in Canon Digital Photo Professional (DPP) using the Standard Picture Style with sharpness set to "1" (0-10 scale).
Note that this setup does not report the reduced aperture and increased focal length to the camera, but the camera continues to work normally.
The adapter modification was not hard to make. Buy a Vello Auto Lens Adapter for Canon EF/EF-S Lens to Canon RF-Mount Camera and follow the steps shared above. I suspect that the greater good of this project was testing the concept. Watch for commercially available adapters designed to fit on a Canon RF extender in your favorite store soon.