My small story with Sigma.
Impressed By the 50mm ART , I bought one for my 70D.
You buy this Lens to shot at 1.4. So I started to shot at 1.4 And... Wow, weird focus issues... You shot at 30m And the focus Is at 40m.
You shot at 3m And the focus Is at 2m. And the next day It changed.
You know the story. AF Is good via Liveview, etc, etc, I think You Have read tons of posts on this like me.
So, I bought the sigma dock to try to fix And after hundreds of shots only one conclusion : inconsistant focus.
Then I decided to send the Lens back. I take a new one. Same issue. I continue to read the forums.
Hmm. Many Poeple fire on Sigma everywhere. But others are very happy. All the data Put into my head I decide to try something : I send my 70d, I buy a 6d.
And... The miracle came
Perfect focus.
So, I dont know what to conclude exactly but one thing Is sure : If You talk correctly to the Lens, It does what You expect. The 6d knows how to talk, the Liveview knows how to talk.
I cant say what happened to me Is what happened to others, but I am sure sometimes Sigma Is wrongly blamed.
And what I can say too Is that this sigma 50mm Is the best Lens I Have ever had.