Inclusion of full viable use of IPTC/EXIF metadata is currently a prime deciding factor in whether my next server will or won't be a Synology device. XMP would be a very desirable plus, but I believe IPTC/EXIF is essential -- lacking it is a substantial compatibility failure, IMO.
Personally I use my current Synology device for local storage as well as making a large number of photos and videos available to family scattered around the world, both old family history material and new/modern stuff. I already use other software for the majority of photo management including tagging, captioning, etc., but PhotoStation allows me to store and share without duplication in a separate management file/folder structure -- but the quality of sharing is severely limited by the lack of proper metadata support. I can't (won't) go through thousands of existing files to duplicate the existing captions to a PhotoStation 'comment', nor am I interested in being locked into PhotoStation by -only- using 'comments' going forward. That would, I believe, be quite senseless.
Otherwise I'm pretty darn pleased with both the simplicity and the features of the device as a whole, and plan to get a more powerful device in the fairly near future. But I'm willing to trade commercial convenience for a DIY device to get such a basic feature set currently available even in free software.