VLC works for the subset of subtitles you, an English speaking user, uses. VLC subtitle support does not support things like ASA or EBU subtitles which are used in other parts of the world. In addition rendering of those subtitles is very constrained. These can be added or configured of course.
Secondary audio is not only additional languages but things like descriptive video services (narration for the blind). VLC does not generally support having multiple active audio tracks without flagging things on the command line.
Arabic/chinese/japanse subtitles work perfectly in VLC work perfectly and look much nicer tahn on Netflix for instance.
I have never seen descriptive video services in any streaming service so I very much doubt it is a major issue, and even in that case why wouldn't you be able to mix it with the main track in a separate channel (which is what is done on TV, as TV does not support multiple active tracks either) ?
Even so, being able to mix the premixed descriptive video track with the normal audio track may still be helpful to users who want different mixing levels than what they used (because sometimes the music is too quiet, for example). (I don't like descriptive video so much myself, but some people do use it, and so may wish to alter the setting.)
Secondary audio is not only additional languages but things like descriptive video services (narration for the blind). VLC does not generally support having multiple active audio tracks without flagging things on the command line.