One thing I noticed is that their library is essentially Backbone and Underscore, and their docs state that it has all existing functionality...except I keep finding functions that aren't included, so I still have to load my own version of Backbone.

But you're right: for my next app, I'll probably just use their REST API directly.

Odd. I wonder why they didn't just override Backbone.sync.

There is some stuff that we want to handle differently, like when you increment a field it is usable locally but maps to an increment operation rather than a set on the server. So for a slew of details like this, overriding Backbone.sync wasn't quite sufficient. I hear the concerns about the trickiness of using plain Backbone with Parse, though - that is a design issue we are thinking hard about.

I think that's exactly what Backlift does: https://www.backlift.com/

(I'm not affiliated with them)