Well, that's already a strong barrier to its use. When creating a GUI for an app, the toolkit must offer bindings for the language you're using, it's not you that needs to learn a bespoke language just to use the toolkit.
> It can produced mobile, web, or desktop apps from the same codebase...
That's the thing - not really. It seems it can produce a web or a mobile app while will _run_ on the desktop. But mobile and web apps do _not_ work as desktop apps, nor the other way around; the whole UI/UX language is different... even if some companies try to foist mobile apps onto desktop users.