The only real downside to the UD is the bouncy floor and the delay to disembarking due to congestion around the stairs. But, UD is much quieter.
That is one of the advantages to LD - everyone prefers UD, makes for a much quieter cabin downstairs, and better, far more attentive service.
I have been on flights where the UD was full, and only about ten of us downstairs, absolutely NO complaints about the service.
Dave