I finally have my thoughts to chime in.
I like this lens, though have to say I won't be using it for probably 70% of my work. Why?
I shoot lots of gigs. Often the light is bad at best, but when I broke out my tamron 24-70 VC I found it could not focus in poor light. It was probably -1EV, but even with the AF assist lamp it was returning drastically OOF images, while acknowledging accurate focus.
The 50 1.4, 85 1.8, sigma 35 1.4 and the Canon 24-70 2.8 I&II all focused like champions in the same conditions even in servo with no assistance
So, in a bizzare turn of events, I find myself unable to use a standard lens designed for low light work in a low light environment. I guess I'll be using it for weddings, portrait shoots etc, but it'll be staying in the gear cupboard for gigs. Which is a shame, really....
If what you are saying is true, I would return/exchange the lens. I have no such issue and after AFMA have consistent focus results on all of my bodies with the Tamron. It shouldn't perform any differently in low light than the other lenses save perhaps the Canon 24-70II. I get much more accurate and consistent results from the Tamron than I do from my own copy of the 85mm f/1.8 (which is actually my most inaccurate focusing lens even after careful/multiple AFMA). It's not terrible, but I have very high sharpness/accuracy expectations for all of my kit.