I don't know much about that, and I don't own the Tamron 17-50mm. I did own one briefly, and didn't notice any AF issues, but returned it due to that fact that it
did not seem to be a true f/2.8 (seemed more like f/3.3). But it seems to be one of the most popular after-market lenses, so just the fact that there are so many copies out there could contribute to a larger volume of complaints. Just imagine how many Tamron 17-50mm lenses are out there compared to the Sigma 16-50mm.
The Tamron 17-50mm is also a popular first lens for newer photographers, so maybe some of the so-called issues would not be issues in the hands of an experienced photographer. Just speculating.
But all that aside, the low price of the Tamron makes it a popular value pick. With the money one saves with the Tamron, they could get a super-wide zoom to go along with it.