That interface could have been built in vanilla nix, though.

Instead, a bunch of very useful features are bundled with flakes, like pure eval, eval caching, and git-awareness. But flakes still have some showstopping usability issues preventing users from benefiting from these great features. Issues like that it copies your repo root to the nix store on every evaluation, which scales terribly to bigger repos. Not to mention other issues like the extremely limited ability to override a flakes inputs - you can't pass a configured instance of nixpkgs for example.

Generally speaking, the more advanced the use case, the more likely it is that flakes won't work well. Which probably helps explain why flakes are still unstable after so many years.