Well, that is a HUGE showstopper....what is the point of it if it doesn't let you sync your progress?Yup, although it's still in beta, and it doesn't even work for content from your own Plex library right now. It sounds like they're working on it, though.
(From https://support.plex.tv/articles/universal-watchlist-beta/)
I will come back to this in 1 years time and see how revolutionary this new feature is....