As someone who up until recently would have agreed with you, both of these are fundamentally familiarity issues. Nix's error messages really aren't that bad, but chances are there's exactly one line out of 200 that tells you what's wrong. Learning to read stack traces for a new language is part of learning that language.

I've not had issues with Nix APIs, at least not Nixpkgs or the language builtins. When something does break for me, it's usually some random JavaScript package that had some external dependency change. Nixpkgs is pretty well organized and I find navigating it not that hard once you read the packaging guidelines. find / fzf / ripgep / etc. are all great at this, as file and folder names are critical to the organization of nixpkgs.

The big turning point for me was trying to build and package a non-trivial application and build a NixOS module for it.