It does not hardcode subs as noted, I'm not sure what it's supposed to do with styling and fonts nor what quicktime/iOS support for such is like, I just know that it allowed me to convert an mkv with subtitles into an .m4v without it taking all day due to not needing to rencode video and lose more detail by doing so. If it does not preserve these to liking, then it is quite possible it's not the tool for you.