Afaik nothing bad can happen. If you don't do anything special, KOReader's icons will show up as files in Nickel. Most people prefer for them not to do that. The problem is that if you then add the .adds folder to the ignore list, Nickel will delete them and you'll have to reinstall KOReader/Plato.
See
https://github.com/koreader/koreader...mportant-notes for how to avoid that.