I'd be cautious about thinking that you'll be able to use it for complex projects without eventually needing to enter the NixOS rabbit hole. This hasn't happened to me yet on a project that I'm using devnenv with, but while using nix flakes I've found weirdness in nipkgs that I wanted to address.

On the other hand, the ability to put non python deps in a place that feels like a venv... feels pretty magical.

Yes for Python it seems to sit between a blurry midpoint of "just use Poetry!" and "just use a container!", but I can see it potentially being useful for a batteries-included devenv in more niche applications like Julia, Elixir, microcontrollers/development boards, etc.