The metal card, and all plastic credit cards as well, comply to an ISO standard ISO/IEC 7813 which defines (metric system) the size (85.60 mm × 53.98 mm) the thickness (0.76 mm) and the corner radius (3.18 mm) of the card.

However, the ISO standard does not define how the machines should pull the card inside. Most manufacturers tested their machines with plastic cards only, some may use plastic rolls to pull the card inside which don't have enough friction on the metal card.

The Amex card is not "poorly designed", only "unusual" for some machine manufacturers. Please note that people complain about ticket machines, never about ATM from financial institutions ...