Technology

Jellyfin requires a reverse proxy or similar to be reachable from outside the network, once that's set up, the usability gap between the two becomes a lot smaller. And Jellyfin does, still, have some benefits over Plex - first and foremost: it doesn't require an active Internet connection and an "ok" from a central server to fully function - it also has fewer restrictions when it comes to sharing content and a better plugin ecosystem.

Again, I think both are highly capable servers and I'm running both in parallel, even after migrating most of my personal use to Plex.

It's the clients where it all falls down, sadly. Jellyfin's are, even after all these years, clunky, ugly and unpleasant. The choice of supported devices and systems is also quite limited. This is where Plex shines: they have a, generally excellent, client for pretty much everything you would ever want to play your media on.