Once they’re federated they have full admin access. So they can see who liked posts, and lots of other info.

Plex is a far better and user friendly version than jellyfin or emby in my experience especially if you want to share to friends. Granted it’s not open source and has gone commercial route so there is the risk it will continue there. But for now I wouldn’t push to move. If jellyfin can get some more app support and continue to develop and be ready for when Plex messes up then it will take off.