They do seem to have had some field problems though BB. They released that strange little balance weight to attach to the clutch, and they redesigned the engine-side clutch half, so it seems to me they had some inkling that all was not well in that system. The B&S engine did not vibrate any more (or less) than other single cylinder engines of the same size, AFAIK, but on their mower it shook things to pieces. However I don't know how many customer complaints they received, how vehement the dissatisfied customers were, or how old the mowers were when the complaints arose.
Taking a long view of it, they made - in earlier days - professional or semi-professional mowers that would last a lifetime. They probably made a conscious decision that they needed to move into home-use mowers, and progressively downgraded the products, more or less in stages. The twin-rail SB45 may have been a step too far to preserve their previous reputation, but it was less dubious a product than the 430 and 590 Diplomats, and as you say, asserting that a home-use mower with a ten year life is unsatisfactory, is not a sustainable argument. The world has seen many organisations decide to cash-in their reputations and move down-market at some point in their histories, and this is just one more example. The irritating aspect though, is that they appear to have saved so little money through their design compromises. On the other hand, people my age tend to say that about many similar cost savings, and if they had achieved what was probably their aim, and increased their volume by a factor of five- or ten-fold when they moved to the home market, the total cost saved could have been substantial.
At about the time of the SB45 the company management could clearly see that the mower buyers were favouring rotary mowers. They had to choose to retreat, or try to compete. At least they chose to try, which I think is to their credit.
