Since yesterday's Workhorse Engine of the Day, the Toyota R, inspired our readership to make so many good suggestions, we feel compelled to continue the series with today's engine. Suggested by commenter Joe_Bloe, the Flathead was built in the United States from 1932 through 1953 and beyond that in other nations, including France and the USSR. Since we've (arbitrarily) decided that "workhorse engine" means that the engine was built for at least 20 years and was exceptionally reliable and/or versatile for its time, this engine definitely belongs here. [Van Pelt Sales], [Wikipedia]
Just what "basic engine design" are you talking about!?
As I think has been pointed out already, there's nothing—nothing at all—that's inherently inferior about a horizontally-opposed engine design. In fact, I can think of two advantages: boxers have perfect harmonic balance, so there's no need for balance shafts no matter how many or few cylinders you have. They also allow for a much lower CoG than a Vee or inline engine of the same displacement and number of cylinders (admittedly, at the expense of being wider).
As for air-cooling, again, it's generally inferior more difficult to make an air-cooled engine operate quietly or meet stringent emissions standards, but there's not anything else inherently wrong with it. How do you explain the scores of 911s that have finished LeMans needing only a good wash and an oil change?