steps out of shadows awkwardly
I tried building a cross-platform GUI game server administration tool using Kivvy, and came away with the newfound impression that it's only as cross-platform as the libraries you use. I was using pexpect which turns out isn't available on windows really, and actually ever only got it to run on os x and Ubuntu..
Also there isn't really a wysiwyg editor that actually works (at least it wasn't a few months ago).
I much prefer Pythonista's approach to interface design. I also believe it's focus on using native iOS UI elements is one of it's strengths and trying to make it cross platform would water that down. The only way I see pythonista being a good fit for cross platform work is if there's a platform specific wrapper made for each individual platform that somehow allows for the use of iOS elements within all platforms which Apple would probably dislike. If you want to do cross platform work just use kivvy and there's sure to be an iOS script editor/launcher at some point.