I can only speak for my experience on Carnival, but the Lido selections were very, very limited, with SOME of the dining room menu offered in the buffet. The Deli and Pizzeria are also open, but everything else, including the Grill, are closed. Room service is also very limited.
Norwegian had more Lido options than Carnival at dinner time when I cruised them two years ago.
I understand (could be wrong) that on some other lines (Princess?), you can get the full dining room menu for room service during dinner hours. Maybe someone else knows for sure?