Some things are much harder in practice than in theory. I recently had to work with a JavaScript project that doesn’t get a lot of maintenance love. It requires a JavaScript version that is no longer part of Nixpkgs. Now try figuring out how to build just that version and also make Yarn use that version. It’s not incredibly hard but the documentation is not obvious to find.

Then go on a very very long walk while your Darwin machine is now compiling node.

Contrast that with something like Volta which just works and takes minutes to setup.

Or take anything in Node with a postInstall script that tries to download binaries.

Even Haskell that ostensibly has good support in Nix has a lot of flaws unless you use some of the third party Haskell tooling such as Haskell.nix

Quite a few Haskell packages are marked as broken and there’s only a single version of every Haskell package in the Nixpkgs Haskell packages. Meaning you will quickly run into lots of errors because of version bound problems.

Then there’s matters such a what if you install a formatter and a linter through Nix as is often suggested in those super neat tutorials on how to quickly set up a dev environment. But now you’re using a version of these tools that’s decoupled from the rest of the project.

The list goes on and on.