Nix Pills are probably the best way to get a deep understanding of Nix and some underpinnings of nixpkgs. I read the pills when I started using Nix in 2018 and never had much difficulty understanding the language or most of nixpkgs.
The biggest thing I've hit is linking issues with Rust -sys crates (ie. C/C++ libraries wrapped). There's some very strange behavior if you include gcc and clang (for example) into the same environment.
My biggest issue with Nix tbh is it adds an annoying step to every random repo I want to clone and build.
