Also, besides the weight, diesels tend to be harder on redrives. And possibly the airframe. Pretty big thumps on those one and two cylinder diesels. I've had a couple over the years. Wouldn't want one in a small plane. Now, if someone finds a reasonable weight diesel thats really well balanced, that would be nice.
I would think some of the mid size military UAVs might have diesel engines (bigger than tiny electric hand launched UAVs, but too small to warrant a turbine). A skipper is (rightfully) very reluctant to allow gasoline aboard a ship, and it is a logistical headache for other tactical units who otherwise are nearly 100% JP-8 or diesel.
Of course, the military is often willing to pay a lot more for an engine than a homebuilder would pay.
Vigilant1 & BJC... You are aware of the dirty secret of US military forces!!
The logistics of fuel, lubricants and other fluids is a huge headache for the DoD logistics. A 'more expensive engine' is tiny price to pay if EVERY engine in theater runs well on exactly the same fuel... typically... now Jet A.
During the Vietnam war the following fuels were in use and supplied by DoD... and likely others...
JP-4 [USAF, USA, USN*]
JP-8 [USN* after carrier fires]
Jet-A or B [contractor jet transports]
Bunker Oil [ships]
OH... and God only knows how may lubricant and miscellaneous fluid types were needed for each vehicle/engine/system and special purpose uses in these categories...