It's strange because Jellyfin claims to be able to extract and convert subtitles on the fly, and even has an option to disable that, but for some reason it's choosing not to do it. There doesn't seem to be a definitive list of what does and doesn't work. You just play things and hope it doesn't trigger the transcoding dance.

Kodi's player does work (to an extremely impressive level, including DVD rips I made years ago), but I'm really not keen on the UI, dating as it does from before Android TV was a thing. All the buttons seem configured just differently enough from everybody else that simple things like checking how long a video has left, to backing out so you can finish watching tomorrow are borderline infuriating. I did get a keybinding editor for it, to make it so pressing back stops the video, but knowing all the functions is something that takes more knowledge of it than I have. I'll probably get it configured eventually, but it's annoying to start with.

The Jellyfin UI on the other hand is great. I love the detail on each film, all the cast and blurb and all that. It's just the player is Exoplayer, and every limitation in that is now Jellyfin's limit too. There does seem to be an option to swap it, but what with? I'm not sure what Kodi uses internally. Maybe it's their own thing.