> Am I missing something about how people are using flakes right now?

Yes. Lots of orgs are using Nix today and need to meet SOC2 or better, and are using many different caches to meet compliance objectives. This is creating a significant performance penalty.

Further, whether you care about the separation or not the onboarding process is simply nicer, because it is tied to the environment you're running in (GHA) not tokens you have to copy paste around.

> If I have a monorepo, flakes are likely off the table right away due to the performance hit.

If you've heard of "lazy trees", that work is being spearheaded by us at DetSys to address this problem :).