That is only partly correct. Devenv.sh misses outputs defining packages compared to flake.nix, i.e. there is no way to deterministically build the project as a

That is only partly correct. Devenv.sh misses outputs defining packages compared to flake.nix, i.e. there is no way to deterministically build the project as a nix package. It's the developer environment devenv competes with. Both can even work in tandem. Devenv for the devenv, flakes for the rest.