Nix doesn't guarantee bit-for-bit reproducibility: That is, two different builds of a Nix package may not produce bit-identical output. (They probably will, because of all the sandboxing, but nothing prevents an arbitrary package from generating random numbers at build time.)

Certainly Nix makes it a lot easier to get reproducibility in practice, though.

It is not. In this talk [1] Eelco Dolstra touches upon that a bit also.

[1] https://www.youtube.com/watch?v=QvH5qU1qBXY&feature=youtu.be