Imo trying to make a single UX that just changes a little bit to suit different device types is a misguided approach. Using a large screen with a mouse and keyboard is a fundamentally different thing to using a phone with a touch screen.

Using the same hardware for both would be super useful, but the software stack from the desktop environment upwards should be entirely different (yes, including most of the applications!)

There are some fuzzy boundaries - e.g. imo Gnome 3 has proven that a single experience can feel good on both a tablet and a single screen laptop with a good track pad. But I think paradoxically you need to take different approaches on different use modes if you want to provide true unity.