Unfortunately, not 100% of Nix packages are reproducible, so using it doesn't guarantee reproducibility. Since the tests don't currently use disorderfs, there could be more issues lurking due to filesystem ordering issues.
If a `make install` invocation gzips manpages instead of the package manager or distro hooks, then you won't have reproducible manpages unless the upstream uses the `-n` switch as well. This is just an example of many small and obscure issues that hide inn different upstream projects.