Until very recently, Couch/Pouch would only sync entire databases, and recommended a "One database per user" approach. This is improving now, but it's still very coarse-grained. Last time I checked, anything that has data that you couldn't shard easily is basically unfit for Couch/Pouch - e.g. any messaging app or a Twitter clone. How would you select which data goes to which user? Or do you just want to put all tweets, globally, on everybody's phone?
All of this is absolutely peanuts on Firebase.
