Inline 6 cylinder engines have always been low vibration. Engineers knew this in the 1920s. It is a function of the geometry. That is why BMW continues to make inline 6ers, but they may be the last holdout. Even Mercedes finally succumbed to the temptation of the V-6 because it is shorter and therefore lets the stylists design a shorter hood. Also, transverse-mounted 6 cyl. engines are always V-6 configuration. The Volvo straight 5 is a remarkably robust design but just can't be as low vibration as a straight 6, so the 850, S70, and V70 need the engine mounts to be in tip-top condition. By the way, a V-12 is even smoother.