I think the reason they were eventually superceded was because of the problems in terms of torque reaction and so on if you tried to make them over a certain size. The great attraction initially was the light weight, and good power to weight ratio. As the first world war progressed it soon became apparent that it didn't really matter how heavy an engine was if it produced sufficient power to give a good power to weight ratio. So they started the war with everybody using the air cooled gnome rhone rotary, by the end they were mostly using hulking great liquid cooled V and W twelves, very much heavier but producing much more power. Fascinating to look at Janes from this period, there is a whole section devoted to the various engines. Captured examples were sent off to the RAE at Farnborough, where they examined every aspect of them, from the design and materials used to bench testing to get performance figures, and oil and fuel consumption. This information was then used to improve our own designs, the liberty engine for example which had many features inspired by the best Benz and Maybach engines.
