This is what ultimately repelled me. I've installed it for a couple weeks, and while I appreciate what it's trying to do, the learning curve was not worth the effort. At least not yet
To enable the use of flakes, you have to use the 'extra-expiremental-features flakes' flag.
Edit: Apparently they are called 'extra-expiremental-features' not 'extra-unstable-features'. Regardless the nix docs explicitly describe them as unstable here
They're "experimental" because the RFC wadn't approved IIRC, but everyone in the community (and the new documentation) talks about amd used the new commands.