Balance shafts are used to counteract vibrations from the intrinsic imbalances of the individual cylinders. If we consider only the primary and secondary balance forces, the following applies:
1 cylinder: primary and secondary shake
2 cylinder, parallel, 180 degree crank; primary/secondary rock, secondary shake
2 cylinder, parallel, pistons in phase: see 1 cylinder
2 cylinder, opposed, 180 degree crank: all forces cancel (bmw motorcycles..)
3 cylinder, parallel, 120 degree crank throws: primary and secondary rocking
3 cylinder, 180 degree crank: primary/secondary shake
4 cylinder, 180 degree crank: secondary shake
4 cylinder 90 degree crank: primary rock
5 cylinder, 72 degree crank throws: primary, secondary rocking
6 cylinder, inline, 120 degree crank throws: smooth
6 cylinder, 90 degree V, common crank throws: primary and secondary rotating couples, uneven firing impulses
6 cylinder 90 degree V, offset crank throws: primary and secondary rotating couples, primary and secondary rocking couples
6 cylinder 60 degree V: primary and secondary rotating couples, primary and secondary rocking couples.
6 cylinder, 180 degree V, opposed pistons: smooth.
7 cylinder inline: primary/secondary rocking couples
8 cylinder inline, 90 degree crank throws: smooth
V-8, 90 degree throws: primary rocking couple (with balance weights on crank ends, smooth operation)
V-8, flat crank: secondary rotational vibration (increased power due to better spaced exhaust pulses in each bank's manifold)
Flat 8, flat crank: smooth
V-10, 90 degree V: primary rotating couple (countered with crank weights), secondary rotating couple
12 (inline or 2 banks) smooth operation
Radial, 2 or more cylinders: smooth operation. Radials are usually odd numbers of cylinders (3 or higher) to get evenly spaced firing impulses.
There is simply not a way to design a V-6 engine to have smooth operation and even firing impulses, regardless of cylinder bank angle (90 vs 60) or arrangement of crank throws, unless you use balance shafts.