With that much trouble, one has to wonder whether the problems are with the lenses or somewhere else ... like the camera or the "driver".
Trust me... It is Sigma. When all of my Canon glass works perfectly in terms of AF and all (4) sigma lenses are all over the place, I think I can rule out operator error.
Optically aside from the one decentered one, they were amazing. The AF though is a whole nother story... Also note, two copies of the Sigma was on one 5D3 and the other two on another. So I can rule out the body as well.
The 35L nails focus. When it misses, it may be slight but usable. The Sigma misses by a mile at times. Makes no sense. With my commercial work I can't risk that just for sharpness. AF is very important. It's not like I didn't try though.... 4 copies over a year or more? It's not like I gave up after one...