Much of the perceived balance & smoothness is in the execution and packaging of any given cylinder layout. Many engines outside of MB will make do with a counterbalance on only one side of a given connecting rod. Some oldies even did without a main bearing in between cyinders. I was impressed with the MB sixes I've seen apart. Tapered counterweights on both sides of every connecting rod. Very stout main bearing saddles. The sides of the block (skirt) come well below the main bearing parting line, adding to the block's stiffness. Oil passages inside each connecting rod to lube the wrist pin & cool the inside of the piston. Steel inserts inside aluminum pistons to control thermal expansion, allowing much smaller piston to bore clearances. All of these details cost time & money to implement, but add significantly to the smoothness the driver feels. As each engine mount seems to have a diffeent part number even though many of them look alike, I'd bet money that they are tuned to each particular engine. This doesn't hurt either.

Much more to an engine than cylinder layout. That's just the beginning.

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Norm in NJ
Next oil change at 230,000miles