I'm new to both, and both are terrible if you ask me, but for different reasons. Where I see plex having a clear UI advantage is where it comes with a native app for that platform, which is less often the case of jellyfin (although it's slowly catching up). Being open source, jellyfin has a clear advantage IMO because with enough traction, the community will be able to do wonderful things (think of winamp skins meets android custom ROM scene, or something to that effect).

And as a new comer having only seen the freemium side of plex, it has really weirded me out in some places (sponsored stuff, stuff of no use to me that I can't disable, locked out stuff, including petty stuff like HDR encoding...) , so much so that I don't see myself trusting them my credit card, and so I might never get to experience the "real thing". That's just how my perception of it is: Plex probably needs me to pay for it to become good, but it won't be that much better (and still have many quirks) to justify it.

To each their own and all that, but for my time, I agree with you Plex still has the edge in UI by a wide margin. The advertised content is super annoying but it is possible to trim it.