I jumped ship due to privacy concerns - I didn't like that my internally-hosted Plex web home had like 12 things blocked by uBlock, and I really like open source software!

Because Jellyfin is plain and simple and does not try to sell me "services" or "content" made by plex. It just plays what is on my drive

Tbh I liked Plex when I used it years ago, then I stopped using it as I went full on Netflix/Amazon Prime out of sheer laziness. Having now come back to self-hosting and acquiring media myself, I went with Jellyfin mostly because it's FOSS and it does everything I need it to. I noticed Plex has some features locked behind a subscription which I don't like the idea of, and iirc there were some privacy issues at some point? So those things made me hesitant to give Plex another go.

Also, I had used a friend's Jellyfin before hosting it myself and I was really impressed with how well it worked on my devices, whereas when I used to use Plex I'd see stuttering/buffering issues from time to time, especially if watching a foreign film for example and the subtitles wouldn't load/render from the .srt file properly.

As for apps... my TV runs Android and the Jellyfin app on that works great, and no problems with the iPad app either. I can't speak for consoles though as I don't use mine any more so have no idea what the JF app is like on those.

I liked jellyfins customization but I had to go back to Plex after it kept fucking up on me constantly. And trying to use the app to stream stuff to my phone was annoying as fuck, always wants the IP address, cant just log in like you can on plex