When the rMB came out in 2015 I bought one at the Apple Store. Took it home, and I immediately noticed 2 defective keys. My Enter key was working like 15% of the time. Took it back to swap it out, didn't think much of it. Figured it was just a lemon.
Took the next one home; this new one not only had a couple of other defective keys, the hinge was creaking significantly. At this point I started believing that there might be some quality control issues so I held out for the Skylake model.
At least in my experience, I have had no issues whatsoever with my 2016 model. No creaking, all the keys are perfect, everything's good. As others have said, it really is just luck of the draw. But I believe Apple QA is smart enough to see the trends of returns for keyboard issues --there had to be thousands in the last year-- and to fix the issue in the manufacturing process.