Balance is a subject that can bring on as much discussion as an oil thread when the right people are addressed! Simple fact, no reciprocating engine can be fully balanced throughout it's full operating range. Even flat opposed configuration engines have their issues and some of the early twins shook more than some single cylinder engines even though most flat twins are fairly smooth by nature. What usually happens is that any given engine can have the rotating assembly balanced very well for certain RPM ranges but it may shake at other speeds. Add combustion impulses to the mix and you get even more vibration then add shaft resonance to the mix and you get additional shakes and shudders.

I use to sit behind a 406 racing engine and one of them had a terrible vibration at parade lap speeds (maybe 4000 RPM or so) though it was extremely smooth at high RPM. During a rebuild I told the machine shop of the condition and they said "no problem"! Even though the rotating assembly had been carefully balanced, different people have different ideas how much weight the crank counter weights should have to balance. He changed the bob weights a bit during re-balance and the engine was then quite smooth though yes, it did shake a bit more at just above idle speeds. The point being that any engine regardless of configuration can have its balance altered to move the vibration points around to different RPM ranges.

Some automotive engines have balance shafts separate from the crankshaft and camshafts. The only purpose of these shafts is to counter vibrations that can't be balanced out of the rotating assembly due to design or configuration. These shafts are timed to the rotating assembly just like a camshaft would be.

Some engine designs can't be balanced well due to their basic configuration .... can you say Harley? You simply can't hook two pistons and rods onto a single crankpin of a narrow V configuration engine and expect smooth operation. But tradition is tradition and the shake lives on.

Just barely touched the surface here as there is much to be read on the subject and even more interpretations of how to apply the knowledge.