If you believe that you can just go to the dining room 24/7 and just get a snack, you've got the wrong idea. And the buffets are not constant.
Now, most of the ones I've been on had a burger/fries/pizza area up by the pool deck (usually the "Lido", IIRC) early on in the "lunch" hours, but often during the off-times, only one might be open. Never been on a European cruise, so I'm not sure about how they do the dining.
One thing we figured out was nice, if it doesn't cost extra, is to order breakfast "room service". (Mrs. ZBDent has CP and is slow to move out of bed in the AM, esp on vacation). That way, you don't have to hurry to get to your breakfast before it shuts down, and you can start your day at your pace (unless you have an excursion).
All in all, we loved our cruises, and even got Mrs. ZBDent's non-travelling cousin to go on one to Alaska (we had a larger cabin), and Momma ZBD is going on one soon, even though she was "seasick" on a river cruise (much different animals).
Looking forward to hearing good things about yours! (One day, we will go on a Eur cruise).