Today we're building a custom API on top of Firebase. We're using Firebase Functions and express to build this API. This gives you full control of the data you want to send over the wire at any moment, and let's you get the best of the benefits of the realtime DB, without any of the limits.
Possible use cases:
Fetch only a subset of records to save bandwidth
Allow users to modify scoped data from other users (moderation, voting)
Keep parts of the DB secret until conditions are met