How many (p)reviews of the Sigma 50mm f/1.4 ART actually report problems with the AF? I read a few but only found this report at TDP. Is it possible that Bryan had bad luck with his copy? Although it was provided by Sigma?
EDIT: Lenstip reports an AF problem specific with the Canon 1Ds MKIII. Maybe they should have tried another FF to see if it is that camera or the lens....
I used to own the 'old' Sigma 50mm f/1.4. I never had AF problems with that one. I got rid of it because I did not like to performance on the FF but loved it on my 7D.
It is possible that production copies will perform outstanding where always you have some copies that are not okay.
Lenstip claims the issues were with the 1Ds3, but non with the 5D3. Bryan reports issues on a 5D3.
Good news is that it's probably a software quirk. I would wait a little before buying the lens, but I'm confident Sigma will release a firmware update soon.
We should also wait to see how widespread is the problem.
It's also interesting to notice that both tested lenses (lenstip and TDP) came from Sigma. Maybe they weren't really retail-ready models and had a beta firmware?