I am a super happy Firebase user from day zero. I've built many production-ready apps with it and will continue to do so. So far the only features missing that will make it perfect and a complete back-end solution are:

1. Server code (something like parse server code or AWS lambda) to run event triggers (eg: run more complex data validation or trigger a mail/sms after a ref is created/updated)

2. Full text search. When I need full text search on my apps I have to spin-up a server (normally nodejs) and build/keep a searchable index. Then I think: if I have to run/maintain this back-end piece I might as well build an API for my app and solve the lack of back-end code as well.

3. I am still not sold on React/Vue/Angular2 so the tried and tested Backbone framework is my weapon of choice. Sadly enough, there is no official library to implement Backbone-Firebase bindings so I find myself hacking patches.