I think this is really nice for very small systems, but anything near the size of a RPi .. I’m unconvinced.
Building a UI toolkit that works well is really, really hard. The old “last 10% takes 90% of the effort” adage is 10x worse for UI frameworks. The last 1% takes 50% of the time IME. I would much, much rather see good integration with an existing and mature technology like QtQuick. It is declarative, provides all sorts of absolutely wonderful features and flexibility, including the ability to inline OpenGL shaders etc. And it works a dream on small-ish devices that can run a moderately modern OS like Linux, including RPi grade hardware.
Sequential programming of UIs is, outside of special cases like low-level game development, done imho. It is not realistic for the average human developer to produce UIs that fit modern expectations with a sequential API. The need to manage interactions between components, animations, data, etc. is just tool complex. When the more-or-less-static UI paradigm of WIMP started to die, that was really the end of sequential UI programming. For that reason, having a declarative UI system, as seen in QtQuick, is currently the best route.
(Prior to my current focus back on distributed systems, I spent ~15 years doing UI/UX R&D at medium and large companies .. the above is borne of that experience 
