All EF glass with the adapter currently and it's flawless! Used my EF 70-200 mk ii today whilst I'm waiting on my 100-400 mk ii to get repaired. Think I'll sell the 100-400, along with my R and some other stuff and get the RF 100-500, whenever that releases. Itching for Sigma and Samyang to release some RF primes too.
I am waiting to try the 100-500 first hand before I dump the 100-400. I use that lens on spot news and feature assignments a lot. I have read if you set your camera to 1/3 stops it acts like a 6.3, not a 7.1. But my shop will lend it to me for a few hours to take on a walk before I buy. So far the 70-200 has been flawless.
After testing this camera on every newspaper job since it was released, I sold or am in the process of selling my 5d 4, 50 1.4 (not a great lens anyway) 35 f2, 28 IS, and a few others to get more RF glass. I have 2 1dx mk 2 bodies, but so far, I prefer the R5. The only aspects that the 1d is better in to date, is battery life (which is AMAZING) and boot up from sleep time. If football ever returns, I think I would trust the 1dX in the rain at a game before the R5 tho, just a feeling.