Plex starts narcing on its own users' anime and X-rated habits with an opt-out service, and it's going terribly

You can't really compare the two. Kodi is a media player. Jellyfin is a media server and has a bunch of clients to access that sever. There's even a kodi addon to connect to Jellyfin. It's called jellycon and I am experimenting with it now.

Different use cases.

They work great together. I have jellyfin as my media server / manager /backend, and kodi on my nvidia shield connected to my TV as my main media player / frontend. In kodi, the jellyfin plugin syncs all metadata from jellyfin to kodi's library, and streams the media from my jellyfin.