Even with old `nix-shell`, you could always use `nix-shell ... --command fish` or whatever it is to launch a different shell. Historically other `nix-shell` wrappers, like `direnv`' Nix integration, have also supported non-bash shells well.
> [I] read somewhere that [nix-shell]'s not really made for this purpose originally, rather for just building packages, bash being only one of the limitations.
Yeah, nix-shell was originally made for debugging Nix builds. The first capability it gained was setting up the build environment of a given package, which equips you with the same compiler, linker configuration, etc., as the package would get if you ran nix-build, or when the package is built on CI/CD or whatever. It even loads the bash functions that the build system invokes so that you can experiment with manually adding steps before and after them and things like that.
But it's gained other capabilities since then, like `nix-shell -p`, whose purpose is a more general try-before-you-buy CLI and magic shebangs. It also has two descendants, `nix shell` which is just about letting you set up software, and `nix develop` which is more oriented toward setting up whole development environments and all the env vars associated with it. Anyway I think that's mostly trivia; it doesn't pose any problems for devbox afaict.
> I tried to experiment myself with nix-shell, but I think it doesn't provide separation on the machine on which you run, it's not a chroot nor a docker container
That's true, and that's really the beauty of it: you can set up complex toolchains and use them as if they were simply part of your normal system, but without worrying that they unwittingly depend on parts of your base system that may be unique to you. Likewise, they don't require any permanent changes to your normal, global environment at all. If you've used Python before, you can think of nix-shell like a generalized venv.
> If you are interested in some level of separation to make sure that a dev environment is "safe" to run on your machine without making side effects to things external to the project
Nix can provide sandboxing for builds, for proper packages. So if you want to make sure your environment is complete, adding to your Nix shell development environment to make a complete package may help you.
But the purpose of shells like this isn't to protect you from running `rm -rf /`, if that's what you're after. It doesn't protect you from dogecoin miners in your `npm install` hooks, if you're just using Nix to provide `nodejs` and then running `npm install` as usual.
What something like this does do is allow you to use all that software without installing it. So if you open up a new session, none of that stuff will be loaded.
Nix can generate container images and VMs for you, though, and that is also one of the things `devbox` can do, if your isolation concerns have more to do with security (disallowing access to your system) than 'purity' (disallowing dependency on your system, not installing things to your system).
I hope that makes sense :)