Cleaning up BACKUP_REMOVE directory
The BACKUP_REMOVE directory in the default folder path contains folders deleted by users (only whole folders, not files or sub-directories of a folder). The folder can be cleaned up by a cron job (or scheduled task on Windows) regularly to avoid consuming too much disk space. The folders in there are no actively managed anymore (means PowerFolder isn't aware of them) and so users have no access to them. Maybe the folder should be emptied once a month.
Cleaning up temporary upload files
Unfortunately PowerFolder has a bug, so sometimes temporary files are left in the /tmp directory (or C:\Windows\Temp) on Windows after a failed upload. The temp directory can be cleaned up by a cron job (or scheduled task on Windows) regularly to avoid consuming too much disk space. Maybe the directory should be emptied once a day.