Except that libraries are written by people, who have assumptions and probably limited budgets, and the sheer inventiveness of hardware engineers at making things like memory ordering different, or even buggy [game consoles are famous for this] . . . well, libraries aren't magic. You have to test them somewhere, and the failures are often subtle, happen sporadically, are hard to reproduce and are very difficult to debug.

I've used them. But in limited areas, where the win is great and they are explicitly called out as portability hazards, with alternate implementations of the areas in case the lock-free stuff goes pear-shaped. And from time to time we look at these things and wonder if the performance gain is (a) real, and (b) worth the headache.