Today React Native Windows depends on RN core, which uses Folly and a number of other libraries (double-conversion, glog, sys).
Some [parts] of these libraries don’t work in UWP context (e.g. calling CreateFile on arbitrary paths).
We don’t build all of Folly, instead we carve out the minimum set of files that we need (and work in UWP).
Every time we take a new drop of Folly, we are opened up to new dependencies of Folly being introduced (e.g. double-conversion, fmt). These are dependencies of Folly but not used by RN or RNW. However, they are needed to build Folly.