> USE flags a la Gentoo are not compatible with Guix because it means > that package expressions are no longer self-contained and rely upon > external global state. That means that builds are no longer > deterministic.
Build determinism is only relevant for binary caching so if someone changes the default USE flags then he's building the packages him/herself. Using different USE flags also would yield a different build script so the mapping from .drv => binary would still be deterministic.
