You will find the Plex SmartTV apps very limited, generally due to limited capabilities of the TV hardware/software or restrictions put in place by the manufacturers.
No TV supports passthrough of TrueHD audio from a TV app. Plex will transcode the audio to another format (usually EAC3). Any Atmos information is lost in the process.
LG & Samsung (and maybe others) no longer support any dts audio format. Plex will transcode dts audio to another format. Check the TV set tech specs or reviews at rtings.com for information on supported audio formats.
Furthermore, some TVs (LG, maybe others) do not support passthrough of dts audio from attached devices (Roku, Shield, etc). Check the reviews at rtings.com, as they test audio passthrough capabilities.
Subtitles support is an issue. Enabling subtitles will force a video transcode in many situations. On LG TVs, enabling PGS, VOBSUB, or ASS subtitles in the Plex app forces a video transcode. If the audio is transcoding (i.e. TrueHD or dts), enabling ANY subtitle forces a video transcode. Samsung behaves similarly. This comes into play when watching 4K HDR media, as the transcoded video is SDR, and any movie with dts audio (the quality of transcoded video is always less than the original).
The Plex AndroidTV app has better subtitle support than LG (webOS) or Samsung (Tizen). It direct plays PGS/VOBSUB/SRT subtitles even if the audio or video is transcoding. ASS subtitle support is poor. Enabling them forces a video transcode.
I have a LG B7 OLED. It has an awesome picture. I also have a Nvidia Shield Pro because of the limited capabilities of the Plex webOS app.
See [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k for additional information.
