As is the case with pretty much all third-party lens manufacturers, many of the lenses use reverse-engineered "compatible" mount designs, rather than designs from official specifications. That can pose a problem from the standpoint of long-term compatibility, and compatibility of established third-party lenses that are older than new camera bodies from major manufacturers like Nikon and Canon. I know for a fact that Canon has not made their EF mount an open specification, and I believe the same is true for the Nikon F mount with its modern electronic connections.
From an optical and built quality standpoint, third-party lenses tend to be good, but they are often not quite as excellent as brand-name lenses. That is a part of the reason for the higher prices for Canon and Nikon lenses, and high quality third-party lenses like Zeiss. There is a tremendous amount of engineering effort that goes into such lenses to maximize IQ, resolution, and build quality, not to mention many of the material costs. You are paying for quality more so than brand name.
Simply put, there are no guarantees when it comes to using a third-party lens, and you can't always expect top of the line quality from a third-party lens.
That said, there are no guarantees they will not work, either, and many people have found Tokina, Tamron, and Sigma lenses to suit their needs perfectly. I wouldn't write them off because of how many returns Amazon has. If you have done your research, have a specific budget, and the rate of failure/rate of return for a given third-party lens is within an acceptable bound (which will probably always be higher than comparable brand-name lenses), I would say give the Tamron lens a try. If you have the option of trying it out in a local store before buying, I would definitely do so. If you want any kind of solid guarantee, such as a good warranty serviceable anywhere in the world, and top-notch quality, then budget can't really be a consideration and I would go with the brand-name Nikon lens.
