Definitely more stable than Arch. Plus, you can easily roll back if something breaks, and you can choose which packages should use the unstable branch while keeping the overall system stable, which I find amazing. I don’t think I’ve ever had a breaking update, which I can’t say about Arch.
The problem I have with Nix is that you can effectively forget about running random programs or GitHub projects. You either package everything the Nix way or nothing works. As a developer and someone who often likes to try stuff out, that’s really annoying. And Nix, the language, is ass, so is the whole build system. Nobody can convince me otherwise.