Flox is a more comprehensive platform, it's proprietary, and it's designed to replace/hide Nix to some extent. Devenv is open-source and basically standalone, and uses the same kind of Nix configuration interface Nix users are used to from NixOS, Home Manager, Nix-Darwin, etc.

I like devenv a lot and use it on my team at work and for personal projects. Nobody on my team is that "into Nix" other than me, but they don't have much trouble setting up new projects with it. LLM agents are pretty good at working with it as well.