maciej wrote:
What Paul said is quite correct.
I have both the ring and no ring adapters and use them, with my R and R5, for all the EF lenses I collected over the years. I also use the adapters with my Sigma 150-600 lens with no problems. I would recommend the Canon branded adapters as I have found them reliable and they connect electrically with the camera. Extension tubes should work the same as they do with your Canon SLR cameras.
FWIW, I bought a Meike (only ~$60) as soon as I got my R5. It's performed perfectly. Works with my Tamrons and even with extension tubes.