Hi Christoph,
You probably want to be aware of these 2 posts:
I don’t have much experience library, but I did use FlaUI once very briefly in my last week of a previous role as an experiment, it work well for the windows app I asked it to interact with and WhiteLibrary didn’t work with that “Metro” app, that was with RobotFramework 3.1.? before RobotFramework existed.
I never used RED, so no idea if it works with that.
Hope this is helpful,
Dave.