Vee V's in line
It's obviously far more complicated than in line v's vee or even flat.
Every engine has a natural harmonic frequency and most designers gear the engine to run in it's smoothest area, away from harmonics. You'll notice a straight 6 is really happy at around 3000, an inline 4 at about 4500.
The Vee angle is also critical, no use having a 90degree V and 6 cylinders and this is the reason for the Buick's balance shaft. Also the reason GM dreamed up that horrible offset big end crank in the Ecotecs, cheaper than making new blocks with a 60 degree Vee, and of course they knew the 60 degree Alloytec was around the corner.
It all comes down to cost. The V6 is just so much cheaper than an equivalent power in line. Compare the crank, mains etc of an Ecotec to an Rb30. The Rb has 7 mains and a massive main bearing cradle, built like a truck and also found in the 1JZ. The Ecotec has 4-side drilled mains and gets additional help from the alloy sump.
As for V12's they suffer from exactly the same frequency problems as any other engine. GM's 4000hp loco engines were continually snapping cranks until they made a different firing order crank to shift the harmonics out of the operating RPM.
Apparently 5-cylinders is smooth as.
