I lost about 8% when using my $60 USD certificate at a hotel in Japan. They treated it exactly as if I were exchanging a wad of crumpled up dollar bills. My arguments had no impact. It's a slimy trick they pull (since they probably get full value from the certs from Accor), but there's nothing constraining their choice of exchange rate that I know of.

Still, some hotels might actually give a reasonable exchange rate. I don't have much experience with Accor, but with SPG and Marriott about 1/3 are fair. I think it's rare you'll lose more than 10%.

(side note: the hotel in Japan ripped me off on the exchange, but then neglected to charge me any tax, also about 8%, which brought me back to even. I don't think it's replicable, though)