I'm using the Art 20/1.4 and 50/1.4 on an R6. Both are excellent and work without hitch. I also have the really old non-OS Sigma 150/2.8 macro, and the pre-Sport Sigma 70-200/2.8 OS, and they work just fine too - much better than they ever did on my 7DII.
I've even used the ancient non-OS Sigma 120-300/2.8 on my R6. I found that lens basically unusable on the 7DII, but actually pretty fair on the R6, between the IBIS and the essentially-perfect AF (plus obviously the far lower pixel density).
Coming back to your question, it seems to me that AF, especially face/eye detection, is enhanced somewhat by the sharpness of the Art lenses across the frame. I have an EF 28/1.8, for example, which is one of the softest primes Canon has made in the relatively modern era, and with that lens I find eye detection etc get flakier towards the frame edge than I'm used to on the R6. It still works of course, it's just that distance-related AF flakiness/hesitation kicks in a bit earlier than it does with sharper lenses. The Art 20/1.4 by contrast is pretty much rock solid in terms of AF on the R6, even wide open close up at the frame edges.
Out of curiosity I did try my old Sigma 30/1.4 on my R6. It's a crop lens, of course, but it covered somewhat more of the frame than I expected, and it did basically work (including eye AF), albeit obviously with extreme vignetting.
Yes the Sigma lenses are very big - that's the reason I keep the EF 28/1.8 around, which forms half of my lightweight out-and-about kit for when I don't want to be weighed down (alongside the EF 85/1.8). Personally I find the Art 20/1.4 and 50/1.4 well within my comfort limit while I'm actually using them, but carrying them around all day is another issue altogether.