MCC is a hex dump that most players, and people, cannot read. ASS gives full control over how subtitles look. MCC is at home in closed captions for broadcast and streaming delivery that carry CEA-708 as well as CEA-608; ASS is the usual choice for anime and karaoke subtitles with fonts, colours and positioning.
The text is read as UTF-8 or UTF-16, whichever the file is. An older file in another code page is read using the charset it declares, or as Windows-1252 if it declares none, so accented letters in an unlabelled legacy file can come out wrong. A file that is not text, such as the picture-based subtitles of a DVD, is refused instead of being converted to nonsense. Of the styling, only italic, bold, underline and the position on screen (top, middle or bottom, left, centre or right) travel between formats; fonts, colours, outlines and effects do not. The CEA-608 caption data inside each packet is read, as for SCC. Captions that exist only as CEA-708 service data are not read. The file is written with a single Default style (Arial, white with a black outline) that the player or Aegisub can restyle, and times are rounded to hundredths of a second, the precision ASS has.