I have half a dozen larger Nitecore lights, starting with the TM11 when it came out. I also had close to a dozen Tubes I used as givaways, a couple of Tip CRIs and a couple of LA10 CRI lights. I have not had any problems with any of them. I also have a couple of MT06 2 AAA penlights that are very nice. I gave one to my GP last year for Christmas, and kept the other one. I am really looking forward to the MT06MD with the third mode and Nichia emitter, and have just ordered a P30.
I think part of the current problem is perception. As the saying goes, if you sell a product that someone likes, they will tell two or three others, but if the have problems with it, they will tell ten or fifteen. Add in the increased audience offered by places like here at CPF and/or some of the flashlight forums on Facebook, and one can get a skewed perception of what is reality. I'm not saying they are perfect, but when you are making hundreds of thousands of a product, a few bad ones are going to slip through. And if some poor soul who buys multiple lights will get more than one of the bad ones. Then they see someone else complain about the problem they had and they chime in as well.
Nitecore does have some problems. Look at the EA4 switch cap issue, the high parasitic drain on the EC4S and EC45S series and you can find areas that could and should have been done better. But the parasitic drain issue is not a quality issue but a design issue, and one can make an argument that the switch cap issue was also a design issue as well. All told, I think most of us are very happy with our Nitecore lights.