First of all there is no option to show/hide the password that is being entered. With a virtual keyboard this is important to avoid mistakes in a complex password. This has been implemented by Android, Windows, and BlackBerry but is still lacking in iOS.
Additionally the keyboard ignores the “Character Preview” setting whenever you are entering a password. Furthermore the keyboard does not use the BlackBerry 10 trick of showing an additional row of numerical keys on password fields. Making the number keys easier to access encourages the use of stronger passwords by making it possible to quickly enter more complex passwords.
None of these are major problems by themselves, but overall you would really have hoped that Apple would be able to put to together a better UX by the tenth version of their operating system.