We're building, and almost live, with a mobile app based Chat product built on Firebase Realtime DB. Firebase definitely made it super easy to build for our use case and we were super happy with having multiple solutions for a mobile app scenario covered from analytics to crashes to auth. However the most critical aspect Auth has given us a lot of trouble. Phone auth in specific, when undertaking OTP via SMS seems to block out numbers it tags as malicious - the problem with this is that more often that not its blocking numbers that are perfectly fine but have logged in too many times. This is just broken and what seems to be an optimisation for cost, is causing a whole lot of troubles during dev and for use cases where multiple logins are required.
If someone from the firebase product team still see this page, please help solve this. This is literally the only thing making us dislike and no trust the platform/