By my understanding:
Inline six counterwieghts are there to reduce crank bending loads and bearing loads. They do not contribute to a smooth running engine in terms of externally felt vibration.
Not true on other engine configurations such as v8. v8 has unbalanced forces and will vibrate externally. The externaly felt vibrations can be altered by changing the recip factor or under/over balance.
For example, balance factors on motorcycle engines are chosen so the engine vibrates fore/aft instead of up/down for rider comfort.
