As far as I can see it is less complex to build simple Java Applications and do package on the different platforms to get the needed binaries of the project. At least for Desktop with JFormdesigner Plugin of with the UI Designer if IntelliJ it is possible (JFromdesigner costs 50 Bucks a year as an IntelliJ Addon and second year around 30 or 40). That would be a toolchain with a modern GUI using FLATLAF UI. And it is described with tons of how to’s in popular internet sites. I guess that would be less complex and not that crime you do with the GTK Components to the users eye’s. But that is only what I mean.
The other side is that you can build a Java Swing Hello world with installs within less then ten minutes on a PC or Mac or a Linux Device.
Speaking about Mobile you can use your Logic and write a mobile App for Android and IOS with CodenameOne which comes integrated with a Simulator.
For Web you can use Vaadin. That has really improved capabilities and security. But: only my impressions. I can not see any good in Vala. Since it is builded for what it is: a Gnome GTK Builder and it is enhanced Swith a few Libs for other OSS’es it is still not really made for cross platform. And it is Definitely a long term tool while the Desktop Development of the Vala community is always for Gnome technologies first.