Better than Docker in terms of reproducibility. While Docker containers are usually more or less reproducible, Docker images are not as the Dockerfiles depend on lots of external state such as the repositories of the distro used as a base image. This is also partially true for NixOS, but it's far more realistic to pin a version of nixpkgs (the Nix(OS) repository) than do the same with Debian repositories. The new Flake format even provides a way to pin nixpkgs by default.