I attended a Canon event in Bristol last Wednesday specifically to use the EOS R, 24/105, 50, 35 and adapter with EF lens. I have to say I was very impressed, however it is different to use to say the 5D, 7D range.
Couple of points raised and answered on the day, the sensor is made specifically for the R and not borrowed from the 5Div. The reason for no IBS is IS is lens specific, bodies with IBS only work well with wide angle lenses. Also some EF lenses are performing better on the R with the adapter, they named the 50L1.2 did give better images.
Not my words/thoughts, just notes from the day I thought good to share [emoji4]