1) better tone mapping, allowing me to watch HDR movies on SDR without it looking bad.
2) Plex, on my variety of clients, had regular issues with audio out of sync. I’m very sensitive to this and it drove me nuts. I have no issues with Jellyfin. I fiddled with all kinds of platform and Plex settings but couldn’t find a solution that solved it for good.
3) It’s easier to access Jellyfin over Tailscale. Plex’s normally clever way of exposing itself to the Internet gets in the way. It’s not impossible though.
Having said all this, Plex has better client UIs by far. They’re well organized, feature rich, and generally bug free. I still run Plex and plan to at least for the DVR functions.