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my SAE engine design book is still packed away so it's hard to document right now, but I believe the flat twin has nowhere near the smoothness of a number of other engines because in addition to the primary and secondary forces of rotating and reciprocating masses you have to remember the uneven firing pulses, which make it not much better than a single. the inline 6 is among the best, the 90 degree V8, V12 and V16 good as well. V6 engines leave a lot to be desired. In addition to BMW using I-6 engines for smoothness and favorable torque characteristics, GM also employs an inline 4.2 liter I-6 (275hp) in their mid-size SUVs. I believe you'll also find an inline 4 is far better than a V4.
In addition to the problem of the length of the I-6 packaging, you also have a long crankshaft that has to deal with the challenges of flex and torsion.
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1974 914/1.8
2005 BMW 530i
2008 GMC Acadia
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