Been balancing stuff for more than 50years Andy. Balance is done for many reasons including internal loads. If you balance each part you do a better job and can inter change parts quickly. Static balance is only suitable where the object has minimal length. Reduction gearing introduces issues direct drive doesn't have. External overall smoothness doesn't mean all is well in there. Counterweight extra shaft rotating engines have Higher bearing loads as the factor is higher, . Weight prohibits their use in aircraft engines (and they are a silly idea anyhow). Cranks in aero engines are as light as possible and don't have the balance weight cars etc do (Excepting Radials) where they are massive with say Nine reciprocating bits hanging off One crankpin. to oppose.. There's also "Torsional " vibrations which you can't feel, that can be destructive at critical RPM's. and put great loads on the reduction gears and crankshaft. Nev