I had an opportunity to tried out the preproduction version of the camera as I was invited by Canon personally to tried it out. They did not pay me to do free advertising here. What I will mentioned here will be solely on my personal opinion and you should take it with a grain of salt.
Somehow I was not allow to share the preproduction image publicly.
First of all, despite of being a small camera similar like 800D, it felt very solid built. It doesn't feel like a toy or consumer camera. I felt quite comfortable holding it for a long period even with 70-200 f2.8 IS.
The came the control and handling. It will take sometimes to relearn as the control was totally different from 80D/5D/1DX.But the good thing is it's can be heavily customized following your preference. The current 5D/1D has pretty limited customization compared to EOS R.
Despite of the missing joystick and the thumbwheel, this was somewhat replaced by the large LCD touchscreen.
The viewfinder is fantastic. The color rendering is very accurate. It's very comfortable for prolong usage i.e whole day event.
It has a great backward compability with existing L lens i.e the focusing speed and accuracy for 70-200 f2.8 IS is as good as in 5D mark IV body.
Using edge focus point with large aperture lens i.e 50mm F1.2 no longer has hit an misses or the need to do micro adjustment. It's dead spot on.
The only drawback was the battery life. It's not as good as 5D or 1D series. Be prepared to get a bunch of batteries if you're shooting for a long period.
Another thing perhaps was your client might wonder or perhaps might ask you "Are you a pro photographer?" as the camera looked small compare to 5D or 1D. Most of the layman who used to large pro body would wonder if they money are well spent.
As for the high ISO, the whole range is very usable even up to 40000, it still can reproduce the color accurately. But I don't think it's good enough to print it for your client.
Low light focusing is amazing, it's a lot better than 5D mark IV!
I think side aside the specification and the miniscule scale of the camera, the whole package was a better camera than 5D mark IV.