I don't remember exactly what Leo said because I don't have his book right in front of me (I do have it though). I know that inline's in general have balance advantages.
I took this section of text off of this website.
http://www.canadiandriver.com/articles/jk/at_010424.htm The piece is on the GM 4200 inline but most of the ideas apply to our inlines because they're talking about cylinder configuration advantages/disadvantages.
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"Sutter described the inherent smoothness of an inline 6-cylinder design. It has both primary and secondary balance. Primary balance is when the crankshaft counterweights offset the weight of the piston and rod. Secondary balance is when the movement of one piston balances the movement of another. V6's have a secondary imbalance that causes engine vibration. Adding a counter balance shaft can reduce this imbalance, but that adds weight and complexity. The inline design was a good choice. The new Vortec 4200 engine runs smoothly, quietly, and quickly all the way to its 6200-rpm redline.
There are some difficulties in designing an inline 6-cylinder. The length of the engine allows more harmful torsional twist in the crankshaft, and makes cooling all the cylinders evenly more difficult. Stiffening the crankshaft helped reduce torsional twisting and a three part front crankshaft damper helps control twisting forces."
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I think the problem with the 292 was in fact the long stroke like you said. But I'm not sure it was harmonics so much as torsional problems. I remember reading that since newer and better dampers were available people were using the 292 more and for higher RPM because it was now capable of it. That's torsional dampening, not harmonic. People love to confuse the two.
So yes, good balance, but torsional problems. Here's a good article on torsional dampers if you're interested.
http://www.grapeaperacing.com/GrapeApeRacing/ttramblings/dampers.pdf my .02 cents anyways. Hope I was helpful.
Greg
