I found what really helps Jellyfin on my Chromecast is setting the player manually. There’s a setting to make it ask which player to use when starting a show and if one doesn’t work, 99% of the time the other one works fine.

Sometimes switching players doesn’t fix subtitles for me, in those situations I usually have to toggle subs a few times or restart the stream and they actually work.

In my opinion it’s a minor enough inconvenience given Jellyfin is 100% free and open source, whereas Plex is tracking you and charging you. But of course maybe your media is in some more difficult format than mine.