This subject comes up regularly.
And your response is the typical objection :-)
When looking at the Thielert and Austro Engine, I find it quite remarkable that those engines are car designs and that the modifications done especially by Thielert make the engine lighter, not heavier/sturdier for operating under a high power regime. And yet they turn out to be rather reliable (even Thielert does do well when looking at the numbers).
The major issue appears to be the power transmission. A gearbox is required for reduction and even if you didn't require it, the high torque of a turbocharged diesel requires some sort of clutch/torsion bar. Thielert got it wrong, Austro seems to have done a better job at a significant weight penalty.