I have a rule with ANY product, no matter who the manufacturer is. Simple - three strikes and you're OUT. If you have three defective units, what does that say about the manufacturer? I don't care if it's Microsoft, Apple, Samsung, whatever - three bad units and enough is enough. That's my opinion.
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..