As I recall the reason internal (or would that be zero
) balance motors are prefered in racing circles over external balance is, "crank whip". Adding weight to either end of a rotating crankshaft increases the whip or wobble of the crank at higher rpm. Removing, or adding, weight toward the center of the crankshaft helps to stabilize the whip at high rpm's.
...theres a physics in law in there some where, mass at the end of a stick or mass in the middle of a stick sort of thing.