HostBill's module uses the email address so if the user changes this on their billing account, the one-touch login breaks.
The username for us needs to be the primary login identifier as that cannot (should not) be able to be changed.
Whilst WHMCS stores the login, if the user then changes their main email in Enhance, it does not sync back so it makes resetting details or sending a reminder more involved then say with cPanel.