I do agree that some copies to have unexpected focusing problems in mirror-mode, but so far I really don't have anything bad to say about the Sigma in Live-View mode with the latest firmware, it simply does what's being told, looked at them at 100%, no difference to Canon lenses whatsoever, and why would it behave any differently on an EOS R, I am sure the forums would already been filled with complaints and I am not seeing that so far.
Yes, the color rendition is more greenish compared to the Canon, but it is certainly delivers very nice-looking images for way, way less money. Good job they are still coming out with EF glass(28/1.4, nice), because, for now, they are certainly being trolled by Canon and Nikon to join their native system.
Of course everyone has a dream, for me it is the EF 35/1.4 II look and optical quality, but with much reduced size and weight, like a Leica 35 Lux with AF