It's both rather mature and completely immature to my understanding. There's a C#-based React Native codebase that's been mature for a while but practically no one uses today unless they have to. There's a brand spanking new, mostly finished C++/WinRT codebase that Microsoft took on building themselves (from scratch). It still has the Beta tag on it the last I checked, many C#-era plugins need extensive rewrites to support it, and it doesn't look like Facebook and the React Native core teams entirely trust it just yet for production development. (On the other hand, it looks like the implication is that the Office team has been dogfooding it from day one, it's roadmap is driven by whatever the Office team needs to ship to production today, and if there is a confidence bar for Microsoft dev tools "the Office team depends on it" has always been an interesting maturity bar.)