`2.` is a big one. I want to be able to filter out crap I'm not interested in. I want to be able to set something as watched that I've already watched outside of your particular service. I want to be able to set something as unwatched because I'm going to do a re-run. I want to be able to set individual episodes as unwatched because I fell asleep with it on and want to actually watch it (maybe a few days down the line). I really want control over the library.
The current model of most (all?) streaming services is just something I don't want to deal with.
I want to like Jellyfin. I've been running it in parallel with Plex for at least 3 or 4 years. It doesn't work well most of the time for me though - mostly client-side issues like bad rendering, issues with subtitles, videos hanging during seek, many others... I could live with some of the issues I've found but every time I hop on JF I run back to Plex. As much as I hate some things Plex have been doing and the lack of troubleshootability when things go wrong, it mostly works most of the time. JF mostly doesn't work most of the time (again, for me - it seems it's excellent for a lot of people!).
Well yes, Jellyfin works well enough for me: installed on a NAS via Docker, accessed from desktops, Android and iOS devices, without issue.
It does seem a little fragile at times and some things are annoying (managing playlists in particular). But the way Plex does business is not acceptable to me, so I didn't even try to install it.
The current model of most (all?) streaming services is just something I don't want to deal with.