Early adapters always face growing pains. For instance, some have screen glue issues. You may return 3 SP3 before you get a good one. MS will correct the issue and late adopters won't likely see the problem.
Some units were probably returned because of heat issues that have for the most part been solved..
True, early adopters do bear the lion's share of problems, but many of the issues we've been hearing about, like yellow tint, bad glue job, power, buttons not responding, etc should not be happening to anyone who is already on their 4th unit. I can understand the wifi issues
to a point (although it's REALLY disconcerting that it's been 3 months since release and there are STILL issues for a lot of people), but for other
repeated production issues, that's another story. If I had 3 bad units, I really don't care who the vendor is - the thing is going back. That would include an iPhone 6 Plus, a Galaxy Note, or a Surface Pro 3.