cross-platform is overstatement.

can I run it on Android? iOS?

no? then 99.999999% of real world users cannot access it. and if it is desktop oly, what is the point? it is no better than web.

The needs of desktop and mobile are different enough that it's extremely difficult to build a UI framework for both that doesn't seriously compromise one paradigm or the other.

I would argue it's one of the main reasons why frameworks like Flutter stuggle with widespread adoption on desktop — it was conceived primarily as mobile-oriented, and so on desktop you're stuck with half-baked third party components for essentials such as datagrids and tree views. WinUI with its mobile heritage in UWP suffers similar problems.

GTK + Adwaita tries to straddle the fence and produces a subpar experience on both sides. Desktop data density is terrible due to mobile-minded button sizes and margins (big touch targets, bloated whitespace to make inadvertant touch interactions less frequent) and desktop-oriented widgets like tree views feel out of place on mobile.

Web is fantastic.