well, I say sigma's QC is probably the worst one out of the major lens companies, maybe You got a really good lens and I didn't
in the last 3 years I was using 3 different sigma lenses: 1 was a total mistake (70-200/2.8), 1 was ok (nothing special, but was playing with it a little bit to compare with the nikon version: 18-200 OS - one of my friends has the sigma and another one has nikon) and the third one is outstanding - 10-20mm
as for tamron lenses, I've never had any problems with them - was using some back in the analog times
Quite possibly, but there was one time that left a bad taste in my mouth about tamron. Maybe I got a really bad lens, but my first 28-75 wouldn't stop down. I returned it, and got another. (Which wasn't all too good, either. Very sticky zooming action, that sounded like it was grinding something.)
I completely believe, and know, that Sigma has a very questionable history in terms of QC. But to tamron's credit, they do offer one of the longest warranties in the industry. I've just had 2/2 lenses that I've purchased from them be duds.