Most full featured Jellyfin Client?

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I found that WebOS Jellyfin Client doesn't support subtitles natively. For example, when subtitles is selected, the server starts to transcode the subtitle stating that it doesn't support it.

The same movie and subtitle works with the Android/FireTV client.

I found that Jellyfin Media Player for PC client is the most full featured client because it seems to allow direct-streaming even though my monitor isn't 4k, and it has more customizable options around play-back/subtitles, etc...

I wanted to get a streamer device like the Roku and was wondering if the Roku client is not like the WebOS and more like the Android or the Jellyfin Media Player for the PC.

WebOS, in my experience, supports a broader range of subs than Roku. Neither supports PGS/DVD/VOB subs. Those picture-based subtitles aren't supported by very many clients natively and must be burned in. Recently, even fairly complex ASS subs have worked just fine with WebOS. In general, picture-based subtitles will require transcoding, with few exceptions (none of which I can speak to personally). Anecdotally, Kodi and the Shield supposedly support native display of all subs, but that's just what folks have said.