Did you use the ef 50/1.2L or the horrible ef 50/1.4? Both are very unreliable lenses. I ownwed both. The SA 50/1.4 plays in a different league. I am sorry for you bad experience for you bad copy but have it replaced or repared is the way to go then ... Now with the eos R its even better. My sigmas are incredible relable on it. Seem to be a very lucky guy.
I've had the ef 50mm f/1.4 USM for years. Not nearly as sharp as the Sigma, for sure, but nails AF quickly and reliably, even after the scorching summer I left it (by mistake!) in my car parked in the driveway.
And I'm not saying Sigma is the only company that doesn't like to talk about AF issues. You mention the ef 50mm f/1.2L USM, perhaps Canon's most controversial lens. I have never used it myself. I'm friends with a journalist professor who uses it more than any other lens and and loves it. I'm friends with high-end wedding and portrait photographers who say it is horrible because of the focus-shift issue, and way too slow to AF at any type of event.
I have never used the ef 50mm 1.2L, but I have studied image after image on the web, and I've watched very clear demonstrations of the focus-shift issue on superb videos. I know that I don't want it because of the focus-shift problems--and because I think the "magic" bokeh can easily look very ordinary or jittery.
Canon? To my knowledge, correct me if I'm wrong, has never clearly stated that the lens has a problem at certain apertures and distances (but might have said something about the lens's unique qualities resulting in compromises). And then there are rumors around the Web that the lens had some sort of stealthy fix after its initial release, a fix which improved the AF problems. But the focus-shift issue still remains.
(For reference, I owned and loved the ef 85mm f/1.2L II for years, then traded it in for the ef 85mm f/1.4L IS to enjoy quicker AF and the image stabilization. I believe Viggo is not as happy with his 85mm f/1.4 as I am with mine. The point is, we want lenses that work for us, regardless of brand. We aren't bashing Sigma because it isn't Canon!)
So, no, Sigma is not the only company that dances around problems with products. But I have tried two copies of the 50mm f/1.4 Art, and I'll never purchase another Art, because, sure enough, it AF's erratically and cannot be trusted when narrow DoF is desired to achieve a look. On the other hand, I have a Sigma 180mm f/2.8 macro that actually focuses as reliably as any Canon lens I've owned, and works beautifully even as a portrait lens. Love it. But that was made before the Art series, which, seemingly, made some algorithm or communication changes to speed AF and instead made it hit-or-miss. Nobody expects a long focal-length macro to be a speed demon, so that might be why mine works so well. Or I won the lottery with that one, who knows?
Because Sigma doesn't publicly discuss any shortcomings.
By the way, nubu, do you often use the Sigma Arts wide-open from within six to ten feet of the subject? Just curious about how you use yours.