I only have experience with Qt, and can tell you that a problem with the current version is the difficulty in getting a native look on mobile. If you want a custom UI, QML is great, but Digia really need to offer native-looking UI components. They will do this for widget-based UIs on Android in the next version I believe, but no word on iOS.
Native UI goes far beyond the look. Another concern is accessibility, especially for blind users. Non-native UIs can be accessible, but only if the toolkit implements each platform's accessibility API, and gets the implementation right, which seems to be very difficult. (For example, last time I checked, Qt's implementation of the relevant accessibility APIs on Windows was deficient enough that the TeamTalk voice chat client offers a "classic" MFC-based client specifically for blind users.)
Other than that it's quite nice.