The current crop of aviation diesels would be more reliable if they ran on Diesel instead of trying to run them on Kerosene
That suggests you have inside information on what is causing the Thielert engine to repeatedly pack up
You may be right; I have no idea.
The debate about car engines in planes is much older than I've been flying. I am not convinced car engines would be reliable in planes. A car engine rarely runs at more than about 30% power. At 75% power most decent cars will go fast enough to land the driver in jail on first offence and few roads are good enough for more than a minute or two of say 140mph. Most of the time the engine is doing say 10-20% power. No wonder they are super reliable. Those that get properly thrashed (rallying, racing) don't last long.
Lycomings are reliable, and are efficient in as much any engine will only deliver the fuel's chemical energy, and if one can run it just after peak EGT that is the best one can get. What Lycos don't have is good thermal design, so they need careful management otherwise one gets cracks... they also suffer from poor QA nowadays.
Diesels in GA stand and fall almost entirely according to fuel taxation. I do long flights into Europe and can see other factors: One can get tax-free AVGAS in most places upon the production of an AOC, levelling the price with Avtur (and any serious operator can get an "AOC" if he wants to). One cannot really get Avgas outside Europe, but avtur is everywhere - this doesn't bother the average Euro PPL who does short burger runs! Spain will sell you tax free Avgas if plane is business owned. Etc.
So the pro-diesel factors are even fewer that one might think.
And given the poor reliability, I can't see why anybody would bother. OK in a twin I suppose.
FADEC on a Lyco would be interesting! A turboprop is THE way to go though.