Since you're writing a thesis on cross-platform frameworks, have you looked at the toolchain that Google used for Inbox? They write their cross-platform non-UI code in Java. On Android, that's native. For iOS, they compile it to Objective-C using a tool they wrote called J2ObjC. For the browser app, they use GWT. The UI code is platform-specific and written in each platform's native language, i.e. Java, ObjC, and JS. They don't even try for cross-platform UI code, as Ray Cromwell explained here:

https://news.ycombinator.com/item?id=8965349

This toolchain doesn't cover UWP, except indirectly via UWP's support for web apps.

Official support, documentation, perf (presumably), debugging, and tooling support would be my main concerns.

Technically, XAML makes heavy use of C#'s events and properties (which C++\Cx adds to C++), which don't exist in plain old JavaScript. Also, both C# and C++\Cx support "await" natively, which is extremely nice for WinRT's async-heavy API's.