The biggest problem IMO is the insanely complicated layout system that is the way it is mostly because it was originally meant to handle long text documents. It took a while to bolt on enough things to have decent equivalents of app-centric approach to layout that desktop frameworks have offered for decades, but it still has to integrate with everything else in the spec, and devs need to know exactly what to look for. Ditto for styling. If you compare either to XAML or QML hands on, it quickly becomes clear just how much easier it is to work with the stuff that was designed for apps from the get go.