I recently joined Bethpage Federal Credit Union to avoid bank fees (especially Bank of America which now charges 3% foreign transaction fee + $5.00 for withdrawals at European ATMs). My BFCU debit card, which is a MasterCard, did not work when I tried to withdraw money from the Belfius ATM at Brussels airport and from the Europabank ATM on Vlamingstraat in Bruges, and I had to fall back to the BofA card. But the BFCU card did work at an ABN Amro in Amsterdam.
When I got home I asked someone at the BFCU branch about it, they asked me: "Did the ATM have a MasterCard logo on it?" I couldn't answer, but just now I checked the ATM finder on MasterCard's website, and the Belfius and Europabank ATMs are both there, with an indication that they are "chip reader enabled" (will read debit and credit cards with "chip and/or magnetic stripe"). My card does not have a chip (BFCU promises they are coming soon) but it does have a mag stripe.
I am going to contact MasterCard about this; in the meantime, does anyone have experience with credit union debit cards in Europe?