It uses FFMpeg vs Plexs proprietary transcoder. So, hardware/GPU transcoding can be done for free.
It also doesn't have the latest "free" stuff Plex has started adding in. Personally, I just want my movies or TV and that's it. If I want steaming or news, I'll go find it elsewhere.
I run both PLEX and Jellyfin in Docker containers on my homelab and I have found Jellyfin to "just work" better. The hardware acceleration from my Intel NUC just works for Jellyfin and was always a struggle in PLEX.
Hardware transcoding in Plex requires the paid Plex Pass or lifetime pass. I'm running Plex/Jellyfin on a 7th gen Intel CPU and until recently had a Quadro card in for hardware transcoding (Picked up a 7700 over the holidays and nixed the dGPU). The server also runs a few VMs and things as needed like PiHole.
Plex will default to a dGPU like the Quadro over Intel Quick Sync on the iGPU and on Windows can't be redirected.
Jellyfin has free transcoding out of the box and you can point it to a GPU of choice like Intel Quick Sync over Nvidia. I'm not 100% if that actually uses the Intel iGPU over the Nvidia card as I removed the dGPU before testing. So far both applications are happy with a few transcodes running on the iGPU with maybe 8-10% CPU usage for each.
It also doesn't have the latest "free" stuff Plex has started adding in. Personally, I just want my movies or TV and that's it. If I want steaming or news, I'll go find it elsewhere.