Thank you for letting me know!
I tested this with the latest version of Iobit Uninstaller Free and I can indeed see one Iobit related folder remaining after uninstalling Iobit Uninstaller with Uninstalr with all the default settings. The folder in question is c:\program files (x86)\Iobit\Iobit Uninstaller. No Iobit related folders remain under Appdata.
I shall fix this for the next version. If you notice the same issue happening when uninstalling any other apps, please let me know which apps and I will ensure these will also be correctly and completely removed with the next version.