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)