The tailcap used to fail in quite some models ... but reports about this are getting less and less now.
Then you have the usual problems of a LuxV light: tint variation and a possible donut hole in the beam. This will not change I think.
Another point is that people seem to like the 18650 cell ... and later versions of the U2 have plastic sleeves and now narrower bodies that prevent the 18650 from entering the light.
There have been reports about dust in the light and on the reflector. I do not know how frequent this is / was.
That is all I can come up with right now.
As you can see, some "problems" are part of the design or the nature of the beast (Luxeon Lottery) whereas the tailcap and dirt things are real issues. Or have been.
There have been other reports on a problem here and there, but non that had consistency to elevate them into a real issue IMHO.
One of my lights for example had an emitter melt and then die ... and was replaced by SF.
I love my U2s. This is my preferred light since it got out some time ago. Did I say I love mine?
bernie