Always nice to see a Cross-platform mobile framework that's not cross-platform itself :) *nix only for now.
Android, iOS, Linux, and OS X are all Unix-like, and Windows is... Windows. There's a ton more work that goes into supporting Windows than a *nix variant.
They don't offer the full POSIX API.
http://facebook.github.io/react-native/docs/linux-windows-su...