I have recently started ripping my UHD Blu-rays for play back in Emby, and discovered issues with subtitle transcoding.
In the past, my 1080p Blu-ray rips have been in HEVC, whatever the main audio track is, as well as only forced/foreign subtitle tracks if there are any. Normally I would extract the PGS (.sup file) and convert to VobSub using BDSup2Sub and that would work fine. This has been a habit that started back when Blu-ray subs weren't supported by the codecs on my old HTPC way back when, but handled DVD subs (VobSub) fine.
The first 4K disc I have ripped that requires a forced subtitle track is Avengers: Infinity War for just one line of dialogue in the whole movie. I have tested playing this back using PGS, VobSub and also SRT. PGS and SRT seem to be OK and everything direct plays, however the VobSub version wants to transcode everything just to burn that single line of text onto the video. The HD audio also gets transcoded back to Dolby Digital.
To test this further, I went back and tried some 1080p Blu-rays I had ripped previously with VobSub subtitle tracks and found that they too are being transcoded. My main benchmark for testing subtitle playback has been my rip of Avatar which used to play fine, but now the subtitles are being burnt in and the audio is also not coming through as DTS-HD as it should. VobSub maintains the font style and colour from the PGS subs on movies such as this that would be lost if I used SRT.
My Emby server doesn't have the processor power for 4K transcoding, so it becomes unwatchable, but is fine with 1080p rips. So for future rips, I will be happy to use PGS as long as it continues to work properly. However, I would definitely prefer to avoid transcoding my older ones with VobSub subtitles so that the audio can be kept in the original HD format. I especially don't want to have to re-mux all of my older movies with different subtitles if it can be avoided either.
Current setup consists of Emby app running on an Nvidia Shield connected via HDMI to a Pioneer receiver (supports HDMI 1.4 and HD audio formats) then to a 4K Sony TV.