Could be. The internet is a great source of rumours, personal 'experiences' bandied about as major issues, and disinformation, as well as an incredible source of good information for all of us who grew up before it came along.
I've never owned a Tamron or a Sigma, but I did have a Vivitar Series 1 zoom back in the 70s. It was quite a good lens for the price. I do have a love affair with the old metal bodied, completely mechanical, Nikkors and use mine on a 30D sometimes. The results are still very good indeed, and they feel solid and smooth.