The person you were replying to was talking about complex subtitle formats. All Roku models only support .srt style subtitles, whereas a lot of anime has .ass/.ssa subtitles encoded and bluray/dvd rips have image-based formats like PGS. Those more advanced/complex formats can’t be played by the Roku directly, so your server needs to transcode the video to burn the subtitles in.

TBH, most content people watch is in 1080p