> The problem is that it lulls you into thinking it serves the purpose well enough and then eventually things break. Then they get more broken. Then you can't figure out which thing you installed is breaking things. Then you finally do and can't figure out how to uninstall the thing that broke things.

I'd rather phrase this as: the 'problem' with `nix-env -i` is "I forgot I did that" or "I didn't know I did that".

I believe since the command is the easiest thing to do with Nix, and many people will try it and maybe got back to non-Nix package management, it disproportionately leads to newbies asking for help; thus, the general mood against it.