One day in April, 2013, I picked up my Sigma 35mmA at my post office (purchased at B&H). I drove straight to an event, put the lens on my 5D3, went outside and took a few shots of a brick wall and a passing train to confirm it functioned. Then I went into a bar, grabbed a lovely bartender, put her next to a window with gentle, soft light and checked focus at f/1.4. Everything was fine. I shot the event with the lens and have never had a problem of any kind with that lens -- it rarely gets disconnected from my 5D3 nowadays.
Now, using it on a T2i was a whole different story. I tried shooting one event using the T2i with the Sigma 35 as a second camera. The OOF rate was in the 50% range. I never used it on the T2i again. The T2i was not at fault, per se, as I used many other lenses on it and all were perfect to acceptable.
For my part, I do not accept any idea of widespread AF issues with the 50mmA -- at least not on FF camera bodies. There will always be lenses, no matter who makes them, that do not work perfectly. If I had to guess, I'll bet the actual faulty AF rate for the Sigma 50mmA is less than 1%. I'd bet the house that I could buy a 50, stick it on my 5D3 and do excellent work.
That's my story, and no one can make me say anything else!!