I have no clue, like you, so I'm just thinking out loud @crisp.
Size, maybe? the Jason app on Android is 40mb, perhaps that's something.
Integration of plugins, maybe? for example, ads?
runtime errors are going to be frequent - There is no editor to tell you if the JSON you're writing is actually valid. I guess there's a short feedback loop when developing, but nothing beats lint errors.
Very limited functionality offline, probably.
Hard to extend beyond the components provided by jasonette, you'll have to dig into the jasonette source code, but this might be alleviated by some plugin functionality of some wort.