As Porsche finally admitted when they abandoned air-cooling in 1998, it's way too hard to regulate engine temperature on an air-cooled engine when you really want to have consistency in performance and emissions. I know with aircraft engines, there are now a few liquid cooled engines coming on the market that are very competitive with air-cooled engines in terms of total mission weight. A lot of the aircooled engines have to be more overbuilt in order to deal with thermal shock or overcooling during low power, high airspeed descent from high altitudes. Liquid cooled engines can simply recirc engine coolant and bypass the radiator by means of the thermostat (how all liquid cooled engines work) and maintain far tighter control of engine temperatures, thus improving reliability in a wider range of conditions.