The problem I have with Nix (and Guix) is that you're shit out of luck if you need it to work together with some network based package manager or existing project that someone didn't add to it yet.

It gets really complicated really fast, and things that "just work" with other packages managers and projects, because they are opinionated and linux-y, most of the time don't work well with Nix or have to be shoehorned into it.

It always feels like you're fighting against everything to get things into Nix that just weren't meant to be.