I am not at all an expert on the Theilert engine, so there could be fact which superceeds what I was told.... However...
Back in June, I was test flying a modified DA-42, including 64 3 minute climbs at various power settings. The right engine seemed to run okay, but at fulll power, as I slowed, and entered the climb, it's power would drop off. at 5000 feet, full power could drop as low as 83% load. when I sped up, it would come up a little, but not to the 100% of the left engine. So I took it for maintenance.
The Diamond service center mechanic gave me a bit of a tour around the engine (as he replaced the offending turbo waste gate controller). He showed me a two conductor electrical connector which was associated with the prop governor. He explained that sometimes the fork type female connectors in there would virbrate to have a larger opening. With this, the signal to the prop could be interrupted. He told me that even a momentary interruption in this signal would result in an instant engine shutdown, and no resart, unless the connector fixed itself.
He said that this was an example, and not the only one.
I really like the way the engine ran, but if that lower level of integrity is the cost, I'd rather have the Lycontinental for the cost....
The SMA diesel has a work around for this, so it will run without power or FADEC. I just wish tha SMA would fill the order I have placed for one!
Diesel and FADEC are the future, but we're not quite there yet....