Why can’t Nixpkgs backend ensure that new package versions build successfully before shipping them to end users?! It would take a lot of work to do so
It's the kind of work that can be automated. Most of the Gentoo tinderbox build-everything-in-the-tree-and-file-bugs-on-failure projects (there have been several over the years) have been run by a single dev. I'd assume that Nix could do the same.
Issues with Gentoo? The only one I've encountered that I didn't sign up for is the occasional difficulty finding documentation for some obscure and unpopular option, like downgrading the freetype interpreter to make it render pixel-aligned fonts the way I want it to, or bridging networks in qemu to allow a VM to address other machines on the LAN without systemd or NetworkManager (still trying to figure that one out). With Gentoo, unlike some other distros, you really need documentation—it isn't going to enumerate the possibilities in a dropdown.
There are, I think, two mutually exclusive XDG APIs for systray icons. They probably implemented the newer one, which should work for most people.