>some online payment systems (especially ones in Canada) have trouble with it.

it's not really anything technical that they "have trouble with", most gateways launched visa debit as an "opt-in" payment method for existing merchants, so if the merchant did not specifically enable it it never got enabled. newer merchant accounts have it enabled by default, so this should sort of work itself out over time. but yes, i don't understand why they made merchants opt in, it's pretty dumb.