Flakes are undeniably useful but it's gated behind an experimental flag and documentation work is ongoing. For now, it just shouldn't be the first point of entry for beginners.
The documentation is the biggest gap for sure. I've only been using Nix for about a year, and I'm using flakes exclusively. Although I don't have direct experience with the legacy system, I could definitely picture it being very frustrating managing inputs via the NIXPATH envvar rather than explicitly with flake locking.