I use NixOS on a server and it is wonderful there where my needs are basic and on the well worn path.
But there's no chance I would want it on my development machine. Even something as simple as installing a python package that isn't in the main repo doesn't have a straightforward solution. There are half a dozen different approaches people take, almost none of them well documented. And most of them involve writing 50 lines of esoteric nix, your own derivation, or a complicated flake (which everyone says to use, but there isn't good documentation). Maybe it is great for those on the inside, but from the outside it looks like a disaster. And that's the last thing I want to deal with when I need a package installed to get my work done.
But there's no chance I would want it on my development machine. Even something as simple as installing a python package that isn't in the main repo doesn't have a straightforward solution. There are half a dozen different approaches people take, almost none of them well documented. And most of them involve writing 50 lines of esoteric nix, your own derivation, or a complicated flake (which everyone says to use, but there isn't good documentation). Maybe it is great for those on the inside, but from the outside it looks like a disaster. And that's the last thing I want to deal with when I need a package installed to get my work done.