I just made the switch for a few reasons.

For background, I was a Lifetime Plex Pass user since it launched, created the POC exploit for token theft (a couple of months before they implemented SSL), and built a clustering/sync application (a few months before they released sync, patterns much?).

I did not think Jellyfin was up to task a few years ago. It is now. All the missing features like themed visuals and audio, chapters, thumbnails on seek, all exist now.

Why I switched:

• API: I have scripts that do different things with different media and they were super easy to recreate with the API. An example would be moving ytdlp videos from my Youtube Watch Later folder to a deletion folder if they've been watched.
• LDAP: I now have user control via my Samba AD.
• Privacy: I never wanted my media list stored with a third party to begin with.
• Plugins: I have a library I tag with filenames, like ==Tag--Tag==filename.ext. It took me a half day to make a Jellyfin plugin that converts these to Genres. It was a nightmare of DB hacking to do it in Plex. Not to mention there are waaaay more existing plugins that are supported. Jellyfin is where this happens now, not Plex.
• Fine grain control: Transcoding settings, bandwidth settings, etc are are open and transparent.