V-10s are dumb because of engine balance considerations. A V-10 will end up with a rotating vibrational couple at twice crank speed. The only way to counter that is with a balance shaft at twice crank speed.
A V-8 with 90 degree crank throws has a residual vibration couple at crank speed; this is countered with weights on either end of the crank. The V-10 also has this residual couple and deals with it the same way.
A V-12 with 60 degree crank throws is basically two inline 6 engines. An inline 6 will run without any primary or secondary imbalances, either vibration or rocking. Therefore, two of them together have no residual primary or secondary couples.
A V-16 is just two V-8s end to end; the residual couples of the 8's cancel. You can also look at it as two straight 8 engines; the straight 8 runs smooth just like the straight 6.
A V-14, just like a V-10, will be plagued by the secondary rotational couple (twice crank speed).
A V6 is just hopeless. You cannot balance all the engine forces; best you can do is a 90 degree engine with a harmonic balancer and a rotational couple at twice crank speed and uneven firing impulses (alternate 30/90 degrees). If you try to even out the firing impulses the imbalances get worse.
For inline engines, an engine of 4 + 2i cylinders where i is a positive integer (6,8,10,12, etc..) will run smoothly.
For V engines, an 8 at 90 or 180 degrees will run smoothly. 8 + 4i cylinders, as above, will run smoothly at any angle between banks.
