Libraries now have multiple "mount points" that are configurable per user. That means I can have a single "Movies" library that points to these folders for me, this folder for my wife, and that folder for my daughter---but it's still just the one library on the backend. I don't need to have a different "Kids Movies" library anymore if I prefer not to.
Can you elaborate on this a bit more?
The way I see it, you still have to segregate the folders/physical location of your media, and then add these individual folders to the library. you can then control user access per folder.
in my case, since I just use Couchpotato to put all movies into a /Movies folder, and just point to this folder in my emby library, the only way for me to control access to specific movies (say to filter out those that may not be as kid friendly) is to use tags
apologies, as this may be exactly what you meant, but just making sure in case there are other ways to control access per user that I may not be as familiar with.
back on topic though - I've never really used Plex so probably can't give a decent comparison. I discovered Emby years ago when I was looking for a way to have a central database that stores watched status/movie pause points across multiple kodi clients. I still prefer to use kodi as the front-end, being a user since XBMC days. Emby did tick all the boxes of what I was looking for and I've been a loyal user since.
I run it off a QNAP NAS, with local and remote clients (on various Android boxes) running Kodi and the Emby for Kodi addon, off the Arctic Zephyr 2 skin. Have about 10 friends streaming off my server and everybody's happy with the fairly intuitive interface and overall solid user experience