So what I'm getting from the thread, this is Plex, minus the cancer, but also minus the features?
And then maybe watch offline support.
Have there been improvements on this front?
Transcoding is a whole can of worms I chose to avoid by playing everything directly.
I would check that your client supports the requisite codecs, and has a sufficient network connection. Or alternatively check that your server has enough horsepower (GPU or CPU) to transcode.
Jellyfin server (when i last tried it 5 years ago) couldn't run on same raspberry pi that hapily ran Plex