Jellyfin is an amazing and underappreciated project. I've run it for a year or so and it's great.

I use (and recommend) the Kodi addon as the client. I had stability issues with the jellyfin Android (TV) app, but Kodi runs on all sorts of platforms and it's very well supported. I use it for music, too (there's a dedicated music player for it in f-droid called "gelli.")

What's the difference between this and Plex?

Jellyfin is free and open source software for one. I don't know the exact difference in the feature set but I have heard people complaining that Plex is focusing more on surveiling it's users to push ads. I just got Jellyfin working with linuxserver.io docker image on my rpi4 and it is pretty nice.

Adding a random Jellyfin anecdote: Jellyfin stable (10.6.2) is buggy and somewhat unusable for me — videos wouldn't play because of some browser compatibility issue.

This seems to have been fixed in the 10.7.0 RCs, and they've been quite wonderful.

The only feature I find lacking is being able to use Jellyfin on a Chromecast, via their iOS app. It seems to be blocked by the lack of upstream SDK support: https://github.com/jellyfin/jellyfin-expo/issues/16

You have to pay for hardware transcoding with plex. Plex (and also Emby for that matter) have more client apps and platforms supported than Jellyfin though, so you might want to check that before settling for one.

Jellyfin is fully open-source and doesn't require any third party authentication. It's not quite feature complete versus Plex but it is very close, and has never been a issue for me or my users.