Just back from a 10 day trip to Greece.
5 in Corfu for a Wedding, then 5 in Athens for some Culture.
Corfu, (like I assume many of the islands) prefer cash in most places and many don’t have Chip/Pin machines, though some of them will just try to tell you they don’t… That said, when using the card there had no problems, my partner used it to pay for a hotel, I used it in the airport, and even in a supermarket contactlessly. Though most Restaurants/Bars were cash only. That said there were plenty of ATM’s to be found.
Athens, much more like my day to day Monzo card usage, worked at most of the shops, cafes, bars, and attractions (Acropolis, Temples, Museums etc.)