My anecdotal experience is: X is more stable/mature than Wayland; NixOS on the desktop is rather stable, but not always perfect; year after year NixOS stability is improving; the most stable distro I tried is OpenSUSE Leap; OpenSUSE Tumbleweed has occasionally problems, like NixOS; in NixOS and in SUSE it is easy to revert to past configurations, so problems can be reduced, but probably affecting security.
Up to date I'm using NixOS, because it is very easy to configure.