The nixos.org website does a bad job at explaining things, but nix != NixOS. nix is a declarative package manager and build system. NixOS is a distribution built on top of it. With nix-shell, you can take a package declaration and enter a shell with build dependencies available therein.

"An entire OS" - that is NixOS. Nix (package manager / build system) can and is often used standalone, on other Linux distributions, and by some on MacOS.

I cannot vouch for ease of use of Nixpkgs' Go building facilities, but at the very least it should be possible to create a necessary environment for development and compilation of a package. Nix guarantees that it is going to be reproducible.

The main downside of using Nix would be that the declarative approach is different from the imperative one - AFAIK, there is no command to just add something to the environment (nix-shell -p does not count as it is a temporary env without a pinned Nixpkgs, so isn't reproducible). The second would be that Nixpkgs seems to only have one version of Go and Co. at a time, so if one needs an older version of something they need to find an older version of Nixpkgs.

edit: as I have looked up, there are actually four versions of Go - "go" is 1.20, and "go_1_*" gives 1.21, 1.19 and 1.18 (on unstable Nixpkgs). I don't know about other pieces of the environment though.