YES - Feature Library can be used in both Part and Sheetmetal environments.
even if not fully understanding the need for a video, since it should be explained enough within help pages
see the attached short vieo.
I am creating cutouts in sheet as well as in aprt files, save them in feature library and use them in every type of document again.
Maybe this is the problem: The features have to be created in ordered or sync in order to work in ordered or sync. IOW you can't use an ordered feature when in sync mode and vice versa.