A phone and a tablet are just wildly different devices. Screen size —sure— but also usage patterns, input methods, workflow conventions, etc.

The “mutationes” that must be made reach deep into the functionality of an app, not just UI-level stuff. The whole purpose of the app should probably change. It’s therefore a big and challenging job to make an app scale seamlessly from one device to another. That’s irreducible complexity. The tools available will always be at best “necessary but not sufficient”.