Complex sentence: They have Visa or MasterCard logos, but cannot be used as "debit cards", only as "credit cards", and they are not accepted by all merchants that accept debit and credit cards, but only by those that accept FSA debit cards.
Split into simpler sentences: They have Visa or MasterCard logos, but cannot be used as "debit cards", only as "credit cards". Furthermore, they are not accepted by all merchants that accept debit and credit cards, but only by those that specifically accept FSA debit cards.