It's designed for embedded systems: it's not so much arbitrary GUI applications as a specific one, in fact the OS is more like a library for a single application than a traditional server or desktop OS. It's not that much unlike a single-process docker container, one where it can be quite hard to draw meaningful security boundaries within it. (you can, of course, run stuff as different UIDs, but generally the application needs to have permission to do basically everything on the system, one way or another).