I like natural keys... if you can prove that they're actually immutable and unique for the thing they're representing. Credit card number is a decent natural key for a table of payment instruments, not for users. Even for a natural-key-believer, users pretty much always need a synthetic ID, because anything you might possibly believe to be constant about humans turns out not to be.
And what if you need to be able to refer to those instruments, e.g. check or use them, but at the same time not expose credit card number to whatever entity needs to do the check?
How do you store the payment instrument for a certain purchase?
