Files On-Demand doesn't sync a bunch of different metadata like tags and datetaken for photos. This means that if you needed to find all photos with no date taken metadata for example you have to download everything, do the search and then free up space.

It would be great if we could sync more metadata for the placeholders. The best scenario would be to just sync all metadata so there's no difference in what we can search for and change with placeholders. If that's not possible either adding common tags for things like photos, music, and documents would be good. Some of OneDrive and Photos' features depend on metadata that the placeholder files don't sync.