For the 'indie' developer, Firebase is a gift. By way of example, I (re)started building an app that tracks anomalies in cryptocurrency volume at the weekend. By building on Firebase (Auth, FireStore, Functions) it's all but ready.
That lack of friction between idea and reality didn't exist too long ago. (Firebase ain't the only ones, Zeit are also doing slightly-different-but-equally-incredible work).
Two points of frustration:
Cost - $0.06 per 100,000 document-reads sounds cheap until you're continuously analyzing streams of market data...but that could simply be a problem with my inefficient code.
Quotas - there's a few minutes delay on removing the quota after upgrading from free to paid plan that shouldn't exist.
Edit: make that three:
The Console - there's quite a few products squeezed into the console and so far it works. But please refrain from becoming like the Google Cloud Platform console which seems to have borrowed design cues from space-shuttle dashboards.