When calling setUser in analytics in iOS this references the native side method as "setUserId" instead of "setUser". If you attempt to call setUser in javascript this will reject the promise with an error.

This is in file react-native-firestack/lib/modules/analytics.js:34

Currently is:
setUser(id, properties={}) {
return promisify('setUserId', FirestackAnalytics)(id, properties);
}

Should be:
setUser(id, properties={}) {
return promisify('setUser', FirestackAnalytics)(id, properties);
}