I think that of all the comments I see on the Tamron that talk of problems of any form - focusing, sharpness, etc - all seem to be from Canon users. And the early reports of focus freezing during tracking and such were all strictly with the Canon mount version. I wonder if the first batches of the lens, which were exclusively in Canon mount, had issues that were either later resolved, or only an issue with Canon bodies that didn't arise on the Nikon or Sony mounts. I know the complaints I hear are very different and in seeming opposition to anything I'm experiencing with this lens - focus is superb, tracking excellent and fast, details are excellent at 600mm even very good at F6.3, definitely no issues with contrast or clarity at full tele, and at least in Florida's decent light, no real issues with the high ISOs (though I'm comfortable shooting at ISO1600 if I'm shooting in early or late day light) - during normal daylight I'm not seeing any real problems shooting at F6.3 to F8 and 600mm, and ISOs of 100-400. I could understand gloomy UK shooters having issues on a grey day - but that's not a Tamron issue, that's an issue of any lens that's not F2.8 or F4, since they need all the light they can get.
I guess if I was a Canon shooter, I would be cautious with the Tamron - testing it thoroughly to make sure I wasn't having the issues others mention - but I see far less issues, or none, raised by Nikon & Sony shooters so far, so that's a good thing if interested in the Tamron and you shoot those mounts.