> make the 90% use cases simple, and make the complicated 10% possible

That's not what we have experienced while using Firebase for the last year.

I have no complaints about storage, but the database is so limited that most projects will become impractical or simply impossible for a number of reasons:

1) No remotely decent querying capabilities

2) No search capabilities other than building your own or relying on third party like Elastic

3) No way to reference data, make joins, etc, like Rethink, Mongo, Arango, or others have.

I'd say the vast majority of projects will need one or more of those points which really doesn't fit into your statement.