Yeah, support for various tablet and phone screen sizes is a core feature. I'm trying to keep it as close to a traditional desktop publishing workflow as possible while still respecting the interactive nature of the medium.

Personally I don't like the built-in autolayout solutions on iOS and Android at all. The latter is too limited, while the constraint system on iOS is over-engineered and difficult to reason about. Since neither platform provides a designer-friendly solution, I feel there's a good opportunity for a third party here.