The modern common rail diesel injection engine is not as reliable as suggested here. They are highly stressed engines, operating at incredibly high fuel pressures to maximise fuel efficiency, and minimise emissions.
The common rail injectors are susceptible to contamination, and injectors are expensive, damaged by even small amounts of water found in fuel. Anyone who has paid a dealer $5,000 to just replace common rail injectors knows this.
Older, non common rail, diesel automotive engines are less stressed, and more reliable, however don’t meet modern emissions targets.
Nissan has just announced it will no longer build diesels, and the Volkswagen debacle shows emissions are not as promised.
Consider the added complexity of a pressurised water cooling system, where applicable, in an aircraft that flies above sea level, a coolant leak is catastrophic.
Common rail desel engines would have replaced avgas already, if they were as reliable is the humble IO 320.