Because the implementation is preliminary, there are some restrictions you must observe, as well as several bugs to be aware of. Here are some of the main restrictions/limitations. (For others, see the General app sharing bugs and X11 app sharing bugs in the bug list).
The "Show in HUD" feature menu item of the Window Menu is very buggy. For a specific list of bugs, search for "Apps-in-HUD" in the bug lists mentioned above.