For any serious app, you still need to have a backend server on the side and your Firebase service often becomes bloated and inefficient. Sometimes you want to store the Firebase data inside your main DB as well and so you end up with two sources of truth and Firebase ends up becoming a third wheel to your project (just a bloated data transport layer).
It's not surprising that Firebase has been sliding in terms of popularity: http://www.alexa.com/siteinfo/firebase.com
It's good for rapid prototyping/MVC but not for any serious use case.
I think the big lesson in the framework/devtools space is that the more opinionated the tooling is, the less flexible it becomes and the fewer use cases it covers.