Some nix language gotchas I’ve experienced while using NixOS personally:

1. Avoid with expressions unless with the inherit keyword for identifying hidden attributes in scope.
2. Avoid the rec keyword unless there’s tracking/control of infinite set recursion (self naming/references).
3. Avoid importing more than 1 instance of <nixpkgs> for any evaluation chain unless there’s magic compute and “infinite” memory.

Tricky is numero three, but crucial for fast feedback regardless of the current thing? I don’t poke around as much to know anymore but; legacy, flakes, community, and possibly others.