A lot. Mp3 is a proprietary format on copyright. Some idiot ceo can came and change the rules, let’s add an ads mandatory for each decoder. Today with a bunch of open source good quality formats, is kind of pointless depending on a private company for your music.

I am very happy using duply, that uses duplicity. Works well both for mounted disks or S3.

I would ignore any recommendations of a sync tool like Dropbox. They will very happy propagate your problem in all your places before you realized it.