There's lots of ways nix can be used. It's framework and shell.nix lets you build any sort of environment you want. E.g. if you don't want to use the nixpkgs ruby, you could install rvm in the env and in the shellhook get the correct ruby. Or use the nixpkgs-ruby as you found out. It's relatively easy to pull any sort of nix libraries into your own .nix file (and even easier with flakes). The most powerful thing about nix is that every derivation is cached, so now you can have your own binary cache shared somewhere, and suddenly everyone's build times go down, especially useful for complicated dev environments.

For example, I cross-compile rust code to arm64, build websites, and deploy the whole infrastructure to AWS including the said rust code as AWS lambdas from a single nix derivation. The cross-compiling toolchains, etc take time to build, but then they are cached and only stuff that gets rebuild are the actual stuff I'm developing / changing. This is the backbone of nix really. You may not think this is not that amazing though, but nix is from ground-up built so that it's very difficult to write anything impure, anything that can't be cached or does not cache properly.

For my work, I've written shell.nix for every project I'm developing. This means my own personal environment stays clean, but for projects I always have the environment I need up in seconds. And if I start running out of space, I can just run nix-collect-garbage and forget about it. This is especially useful for me, as I do contract work and don't use the work PC just for one company.

I haven't actually yet used NixOS, but if I ever have to spin up new servers, I'm considering it.. I have ryzen machine that I use for gaming and sometimes building larger stuff... and I'm feeling like I want to migrate that to NixOS.. maybe someday .. :)

Also if you are an mac user, you really should do yourself a favor and switch away from homebrew