Personally, I'm not using Plex (opted for Kodi/SPMC + Exodus) so it saves me from squandering resources. There are a few little features specific to Booksonic. Quoting from chromecastapps.mobi/booksonic-audiobook-streamer, they are:
"Book descriptions, either you add your own to the server or the app will try and look it up on google books
Both author and narrator are displayed on the book view.
Sleep timer (this is in DSub as well but hidden inside a menu)"
It's not life or death, lol, but for folks like me, it's nice to have alternatives