I get to drive V12 powered vehicles with some regularity. From various Ferrari offerings, to Mercedes, BMW and others, including a good number of older vehicles with V12's.
It's hard to dispute the smoothness. As a general rule, you know it when a V12 is powering your car. The starter motor cranks smoothly with none of the variable speed "sound" during cranking. From there, the smoothness is evident. Under heavy throttle, the typical V12 simply sounds and feels wonderful.
Yes, I understand V8's can be as smooth too, but as a general rule, they don't match the feeling of refinement a V12 has. And, I think that when it comes down to it, that's all it is. "a feeling" of refinement. Mostly due to the smooth sound of more cylinders.