Autofocus performance is in general a mysterious and frustrating topic. I have had a Sigma 50mm F/1.4 for around 3-4 years. I got it to use on my 5D classic, but it front focussed enough to be annoying and there is no micro-adjust on that body. (Canon calibrated the body for me shortly after I bought it used.) I managed to get some use out of it for portraits by focusing on the subjects ear, then the eyes would be sharp!
When I got my 5D III last Fall, the Sigma worked great with it, needing very little MFA. I use it a lot now for full body portraits at about F/2.0 to F2.8 to get just the right background blur. Sorry I don't have any acceptable images to post right now, most of my work is not appropriate for this site.