Another option to check out is Couchbase Mobile. Open source, self-hosted database with full offline functionality and a solid sync solution.

I think Firebase is pretty amazing. Have been a fan since I met some of the team (I think it was at the 2011 Launch conference). Like anything, though, there are tradeoffs.

Briefly, I'd say that there's overlap, with Couchbase Mobile tending to shine toward the more complex end (including making the 10% much easier), and also being extremely easy to use as a substitute for SQLite/Core.

(I work for Couchbase.)