We were on the Prima last month, London (Southampton) to Reykjavik. And yes the ship would be better for a warmer climate cruise, but we still enjoyed our cruise immensely. And we don't understand the criticisms of the buffet layout. We thought it was fine, and also never had a problem finding seating generally though leaving the immediate buffet area for seats.
Glad they went smaller. Our last NCL cruise was on the Epic, and it was much too large. The other cruise lines seem to be going in the opposite direction, building bigger and bigger ships.