I run Plex and am pretty happy. Will likely eventually switch to Jellyfin as Plex is getting lamer and lamer.
Jellyfin's worst aspect is the opinionated file structure. You have to set up folders the way it wants, and then the resulting UI browser is what-you-see-is-what-you-get. Pretty sure it's done this way for automated metadata discovery.
Ideally, this would be designed in two parts: separate the file structure from the metadata discovery mechanism.
I personally want a file structure managed by the OS. Let me make folders and nested subfolders to whatever structure I prefer.
Then make the metadata discovery slightly more manual. Click a media file, click a hypothetical "add metadata" button, and then a simple search box with "is this your movie?" and click apply to import metadata from a search result. easy peasy.
The UI is clearly meant to resemble a typical media app but falls short if the end user prefers, for example, foobar2000's UI.
I’ve been a Plex user since the early days. I currently run it on a Synology NAS in a container, using the Plex app on the AppleTV as my primary client device. I tried setting up a Jellyfin container a few months ago as I’ve been concerned about the direction of Plex. It went poorly.
I have a fairly large library, which Plex never seemed to care about. Jellyfin choked. It took forever to go through it all, and I seem to remember questioning of it was working; it wasn’t clear. Plex on the other hand makes it pretty entertaining to watch covers flip over as the metadata is loaded in to see the progress. Then every app I tried on the AppleTV also seemed to have trouble. The one that worked best had to create its own local cache of everything, which required I spend hours browsing to every screen and waiting before it became reasonably smooth. After that, the layout was still pretty strange. I think it would have worked just as well to point it at a file share. Actually playing videos was hit and miss in every app I tried.
I’m still using Plex. If I need to move to Jellyfin at some point, I feel like I’ll need to build a server with a lot more power than Plex requires. Of course that’s just a theory… a theory that will be expensive to test.
For all the fanfare Jellyfin gets online, I expected it to be better. It made me question how honest the people pushing it are. But maybe they have small libraries or only tested it with 5 movies for the review. I don’t think that’s a real-world experience.