Nix the language has issues for sure (error messages for starters), but "complexity" isn't really one of them IMO. It's quite a small and simple language.

The complexity comes in with the ecosystem and nix-the-build-system, each of which has a dozen different ways to do any given thing, all of which are in active use simultaneously. Some are pretty baffling, like nixpkgs' dual role as both the main package repository and the (almost entirely undocumented) de facto standard library