I recently changed my computer and set up my three nextcloud accounts. Unfortunately I did not specify different directories for each account. As result, all accounts got mixed up and all files where shared over all accounts.
So the client should check and warn the user, if he sets up a new account and the new account want’s to use a local directory, which is already used by another account and for default should suggest another directory.
Fortunately I had a fresh backup of my sync folders, so I could fix it manually.