SMI is HTML-like and many players outside Windows and Korea do not read it. SBV imports directly into YouTube Studio. SMI is at home in subtitles for Korean-market players and multilingual files; SBV is the usual choice for uploading and downloading YouTube captions.
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. A SAMI file can hold several languages; only the first one found is read. YouTube's SBV has no styling and no positions, so the result holds the text and times only.