Crazy how that doesn't at all even address the problem of subtitle sync! It just pastes subtitles as-is in there. What if the subtitle files are at a different framerste? What if the subtitles have the wrong starting offset for the media? What if the subtitles have 1-2 mistakes in them as far as timing?
Hence why there are a dozen subtitle syncing tool projects supplementing ffmpeg like ffsubsync, subsync, alass, autosubsync, srtsync, etc...