I have to question if the good/bad copy isn't shooting technique at least sometimes.
I'm not experienced enough to say yes or no to this, but my Tamron on the Canon was also good.
Poor QA with items in a mostly enthusiast market where people have high quality expectations (and will complain if something is wrong), would probably cause undesirably high cost on the after-sales end.
I think that while QA at a manufacturer producing for the lower price end may not be as diligent as at a high end manufacturer, the manufacturer would in his own best economical interest try to minimize customer complaints and warranty/guarantee issues and the resulting cost.