I've had a couple outages due to major version upgrades: the worst was the major version update that introduced systemd, but I don't think I've ever irreparably lost a box. The main reason I like nixos now is:
1) nix means I have to install a lot fewer packages globally, which prevents accidentally using the wrong version of a package in a project.
2) I like having a version controlled record of what my systems look like (and I actually like the nix language)
I prefer to isolate my development environment already in various ways (virtualenv, containers or VM depending on the project) so I don't need that parts of NixOS. My systems are already run on a well-curated set of software. Two decades allowed me to fine tune that aspect pretty well.
While I understand the gravitas of NixOS, that modus operandi just is not for me. I'm happy and fine with my traditional way.
However, as I said, I understand and respect who use NixOS. I just don't share the same perspective and ideas. Hope it never breaks on you.
1) nix means I have to install a lot fewer packages globally, which prevents accidentally using the wrong version of a package in a project.
2) I like having a version controlled record of what my systems look like (and I actually like the nix language)