"... you can remux from mkv to mp4 very easily with ffmpeg directly."
According to the HandBrake docs (ISTR) mp4 can't handle multiple languages and subtitle sets, so the conversion mkv->mp4 is (potentially) lossy. I'm no expert, just trying to keep the language/subtitle sets I want, maybe I've missed something here. What I do so far is HB encode to Matroska, not MPEG-4. Then I don't lose any of the ones I want. Also I have noticed sometimes MakeMKV is not entirely inclusive in its defaults and I have to add extra languages/subtitles.
MP4 can handles up to 255 audio tracks, and 255 subtitle tracks. Maybe some players only support a single track of each, but the built-in players on Mac, iOS, and Windows support it out of the box. It only officially supports a few audio codecs, and one subtitle format (not SSA or SRT).
MP4 is the basis for HTTP Live Streaming (HLS) and multiple audio tracks and subtitles/closed captions are required for something like that.
Why would I care what the "built-in players on Mac, iOS, and Windows" support? I don't use those. But I do have a very rich media environment.
So neither comment "responding" to my comment actually responds to my query, which is about MakeMKV -> HandBrake and subtitles/language tracks. My very tentative assertion seems to be unfortunately true.
I'm going to have to add that I'm not missing any life happiness by not using the "built-in players on Mac, iOS, and Windows", and my library is better than I can stream. You'll need to kill public libraries to degrade the lives of people like me. I'm quite certain that you've got funding to do that, soul crushing media bigcorp lickspittle tools. Yer basically destructive nanite greymatter digestors at this point.
According to the HandBrake docs (ISTR) mp4 can't handle multiple languages and subtitle sets, so the conversion mkv->mp4 is (potentially) lossy. I'm no expert, just trying to keep the language/subtitle sets I want, maybe I've missed something here. What I do so far is HB encode to Matroska, not MPEG-4. Then I don't lose any of the ones I want. Also I have noticed sometimes MakeMKV is not entirely inclusive in its defaults and I have to add extra languages/subtitles.