Currently the Plex client video player on my Samsung Smart TV (2019 TizenOS model) dosen’t like MKV video files with the OPUS audio codec, so it dosen’t direct play these files and instead transcodes them. But as I understood looking at the codec support capability specs for newer Tizen Samsung TVs, the OPUS codecs shouldn’t be a problem for Plex to support natively on Samsung TVs 
As for PGS subtitles, I understand this is currently not possible to direct play on any LG og Samsung Smart TV Plex client because of limitations from LG and Samsungs side, but I just hope this will be possible in the future.
