In terms of alternatives: I've worked with Firebase before, which is pretty reasonable after you wrap your head around the learning curve.
Data syncing is hard and so far i've only seen two possibilities : roll your own solution using the fact that you have a knowledge of your business that will help you be smart and small, or use a generic solution but take some time to understand deeply its way of working before you build your real product.
I actually had to cancel what i thought would be a short angular/firebase project after two days struggling to sync one big nested data structure displayed on a page, and realizing i actually had to rethink my model from scratch to fit the technology (which ended up being completely overkill for my need anyway).
Firebase does work offline if your app loses network connectivity temporarily (details here: https://www.firebase.com/docs/web/guide/offline-capabilities...). All data written while offline is stored in memory and is synced to the Firebase servers when your client reconnects. We’re working on improving offline support when your app goes into the background, and we currently have a beta version of this available for our iOS (https://groups.google.com/d/msg/firebase-talk/dOocEtjQz4w/ss...) and Android (https://groups.google.com/d/msg/firebase-talk/gYlnLgQ-Yhk/It...) SDKs.