I've bought my first 4K TV and tried to view my videos. And Plex App constantly stutter and saying that my connection is not enough (seem like TV were not able to decode fast enough).
But Jellyfin showed everything instantly and without stutter.
Deleted Plex - as there is no reason to maintain both.
The only real issue I've seen with Jellyfin has been some stuttering on older clients (specifically, very old and crappy webOS TVs). I'm not sure whether this is a weak hardware problem, a network problem, or something else. Other than that, it's been very smooth.
(Context: I was a Plex user and self-hoster for about 7 years, and switched to self-hosted Jellyfin about 2 years ago.)
Subtitles don't work for the most part though, so a bit of a non-starter here as I sometimes have to be quiet, and my SO has a slight hearing loss making dialog difficult to hear.
This is very much a YMMV situation. They work for me, both in the web UI, the native app, and in the Kodi plugin. I think it may have to do with where your subtitles are coming from and/or how something is configured.
If you have a device that can run it, I find that Kodi is the best frontend for JF. It has a lot of extra features that the normal client doesn't.
I have had that issue from time to time. What I did to solve that was to install plugin to extract the subtitle. Now it works perfectly (on webos tv client at least).