Rien bunq offers Maestro cards and Mastercard cards. Whether you call them debit cards or credit cards, well that's up to you. These terms don't have a sharp definition.
If the merchants tells you that you need to use a Mastercard-branded card and Maestro is not supported, then you would need to use the bunq Mastercard in order to pay there. As a bunq customer, the payment experience and the way that the money is deducted from your account doesn't differ between Maestro and Mastercard.
