Firebase is awesome! Realtime has become a great selling point with my clients (and a necessity for things like chat apps). However, after doing over 20 apps with Firebase I've had to combine with other PAAS due to some features missing/lacking: server-side code (event-driven like Parse had or like AWS Lambda) and better/easier search features. In the second case for instance, let's imagine a support app, it makes more sense to use a traditional SQL DB for objects like tickets, customers, etc while using Firebase for a realtime chat inside a ticket. Even a hosted Non-SQL with some good collection search feature is a better fit rather than using Firebase.