Sure, Nix is extremely flexible in input definition, but it's different in the sense that Homebrew exposes a single command to e.g. install a cask from a user-inputted GitHub repository. So all an attacker needs to do is typo squat or take control of the GitHub repository that people are using to install a certain cask.

In Nix, flakes are pure functions and run in pure evaluation mode. One needs to consciously add a Git repository (URL + commit hash, branch, or tag) and then make use of something malicious exported by the input. But to find out what the flake exposes at all, reading the flake (or its documentation) is pretty much necessary.