iOS support is the big one, which I’m working on right now. The other piece is that until I’ve fully figured out all targets, I’m not making any guarantees around backward-compatibility. The core library API is quite stable already, and the general way you would structure your app to use Ballast isn’t going to change, but there may be some minor changes to function names or things like that.
But for other targets, if you’re fine with potentially having to make some minor tweaks, I’d say that Ballast is stable enough to be used in a production app right now. My team recently released an Android app to production which is entirely based on Ballast, and it’s performing very well with no issues reported from the library itself.