PW Ratio
As I recall, there was really one good reasons for the short popularity of these types of engines in WWI. The power to weight ratio of these types of engines was among the best possible for the era. Considering the power levels of the WWI aircraft, every bit of weight reduction helped. As one of the pages posted here points out, the largest engine used in a Sopwith Camel was 130HP.
Beyond a certain point the gyroscopic force problem became uncontrolable and people stopped using this kind of engine.
Jay
