>If it's not as simple as containers, it's not going to replace them.
Nix is more (or less) than a container. It depends, what you are targeting with your nix-project. You can either build a minimal NixOS, that has only a kernel, the minimum required user land and your project and its dependencies.
Or build a stand alone docker (etc.) image and spin it up.
Or you can use the nix shell to replicate just the dependencies of your project in that sub shell for other developers or testing.
I just recently saw a convincing video about nix, comparing it to docker.