The big advantage is a) ability to tweak syncing and, b) DSP for each separate zone / room. I also like Roon and I found it is the only solution for fine tuning a multi-room audio system. You can try both for free, run them side by side on the same machine (I do on an old 2011 i5). Plex is considerably cheaper, has a nice 10 foot TV interface that can be used for music control- Roon has ability to cast to Chromcast for display of now playing but not for any controlling of music.

This really shouldn’t be a show stopper for me as I can’t hear a difference between the two but, hey, I paid money for my DAC so I want to use it!

Plex does not provide an avenue for selecting a USB DAC just for music playback and HDMI for video without manually changing outputs each time. Roon does not yet work outside of your local network (it is in their roadmap however) without highly specific VPN workarounds.

iTunes as a server/remote setup is awfully clunky and not at all consistent across desktop/tablet/phone screen which Roon and Plex do quite well. I purchased it due to its fantastic server/remote architecture plus its metadata and hotlinking within the app to other artists, albums etc. I prefer Plex’s interface to Tidal’s and of course I have my own library of music so having them blended together works well. Works really really well plus it now provides access to Tidal too. Once home internet services here gained good enough upload speed I switched to Plex for remote listening. I didn’t use Plex seriously for music until recently I used iTunes for local playback and iTunes Match for 'cloud' access to my library for listening away from the house via my phone. Have been use Plex since it was OSXBMC and Roon for 3 years.

Musicbee gracenote