Can't speak to OP's reasons, and I am currently torn whether to switch over to jellyfin or not. I am currently running both Plex and Jellyfin side by side with the same media libraries.

I am annoyed by the fact that Plex requires that you log in and authenticate against Plex's own servers to view your own media. I know technically you can set up local access if Plex's servers are not available, but you don't get user account restrictions, watch history, etc when you are not able to authenticate against Plex's servers. If I have an experience that is that degraded, then I consider the service useless unless you are able to access Plex's servers.

Plex's recommendations are stupid and annoying. I am running Plex because I want an easy way to enjoy my own media. The other night, my wife was looking to watch a TV series that I have on my Plex server, which I have watched from my server in the past. When she searched for it, Plex was attempting to get her to watch the series on Hulu (a service that not only do we not subscribe to, but is not even available in our country). We ended up having to browse our TV Library to find it since no search I did would find it.

Plex's recent moves toward monitization are concerning. I understand that Plex is a private company and needs to make money. I am not a fan of the way that they appear to be doing it. It seems like Plex likely will (if they are not already) sell search and watch data to the bigger streaming giants. Maybe the gave away too many lifetime Plexpasses?

That's my two cents.