> I could be wrong but I think nix is focused more on just the producing the artifact part.

That is only true to a point. Nix has an extremely broad definition of what constitutes a "build", in practice, and this process can include things like tests.

That having been said, there are undoubtedly certain tasks that Nix itself is not great at - but when you think of Nix more as a system primitive for expressing build processes rather than as a universal do-everything tool, that just means you would build tooling on top of it for those tasks, like Hydra does for CI for example.

That way everybody wins; you don't need to reinvent the build orchestration parts of the process, it interoperates with the entire existing package set, and the broader Nix community gains a new tool that helps them solve their problems more effectively.