For which type of media do they provide fields? Are they in line with PB Core standards for Audio - Video type of content? At the moment, as I understand it, you can upload all kind of media, and basically describe those, but not represent them at the website. For example: if I would like to register a movie archive, and build exhibits with them, Omeka is not yet offering a solution? Or am I wrong?
Dublin Core is a more general metadata standard than PBCore, but it looks like much of PBCore can be mapped onto corresponding elements in Dublin Core.
It's also possible to create a new element set for Omeka to include PBCore, though it wouldn't carry the hierarchy of elements in the PBCore schema.
Any media type can be shown, though results sometimes vary depending on the particulars of mime types.
Hope that helps