Sorry to bring this thread back up, but I didn't get to share my insight
I was a Plex server user (just let it sit in the background for transcoding when away from home) and now an MB3 user (everything stays synced
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Things that I think MB3 as a server does better than Plex
----Auto artwork (more than just the poster and plot, but disc art and clear art/logo as well)
----MB3 add-on uses almost all of Kodi's library functions (actors, plot, disc art, clear art) with modified skins plus more skins to choose from vs Amber-only
----Multi-users is extremely simple to add to MB3, no plex pass needed. Sharing your plex server seemed nice but it looks really poor in the plex apps and web clients. Your "shared" servers never populate in your standard libraries, you had to navigate into servers first. Having multi-users is really nice to keep track of everyone's watched status and resume points separately (great for relatives and friends)
----Access through a VPN is easier with MB3. By this I mean if you have a private-tunnel vpn client running (openvpn or commercial) on your server then Plex won't publish. There is a good windows workaround for those using windows plex servers created by XFlak in the plexforums. However, if you want to make your own workaround with iptables and openvpn in linux then . . . good luck. In MB3, you just have to forward a single port through your VPN then NAT that back to 8096 on your MB3 server
There are some things Plex still does better as well
----NAS support. I don't think people really get that great of an experience this way without a very powerful nas or they are only streaming around the house, but none the less Plex has the most NAS distributions supported
----PlexNewTranscoder does a good job at throttling and copying when needed. I think the ffmpeg transcoding in mb3 is good, but it works differently than Plex. It's really apples to oranges, but essentially the Plex version works better with lower power servers (for the time being)
----PlexSync - available now and works, requires the pass though
Reasons why I prefer MB3 over Kodi for a server (as they exist today)
----Kodi auto-update can be scheduled, but sending json requests on a schedule in windows wasn't easy enough for me to do. Library watchdog worked a lot of the time, but eventually was removed (seems to lock things up every few months)
----AD has to be called after an update, even when the art already existed in the movie folder
----MySQL always gave me "trepidation" against updating. All clients to Frodo, all clients to Gotham, etc. With MB3 managing the metadata for me and my library populated by the add-on I don't have any issue running the old Gotham Retroplayer OE build in the same LAN with an ARM-Helix version -- watched status and resume points all stored in MB3, so no compatibility issues. I don't know if there was MyVideos compatibility issue for MySQL users this time around with Helix or not, because I didn't have to care