It's not statistically waterproof, but in the almost six years that I've been a member of my club, we've had four in-flight failures of the 1.7 Thielerts (three of which ended up in a field) and zero Lycomings/Continentals failures.
At least, those are the in-flight failures that I know about. I've also had a "computer says no" situation with the DA-40 at Duxford, leading to an abandoned flight. But I don't know how many other diesel or avgas flights were abandoned due to start/runup failures or 'computer says no'.
Our fleet consists of 2 Robin Ecoflyers (with the 1.7 Thielert, now all upgraded to the 2.0), 1 DA-40 TDI (with the 1.7 Thielert, now sold, unfortunately), 3 other avgas Robins, 5 avgas PA28-161s, 2 avgas C172s, 3 avgas C152s and a few other, avgas powered aircraft.
But the odd thing is that none of the four failures had anything in common. Here's what I remember the failures were:
- 1 failure of an oil spray nozzle leading to a lack of cooling of the piston head.
- 1 failure of the gearbox (slip ring or something like that)
- 1 failure of the electric wiring to the fuel pump
- 1 FADEC test button short-circuiting in-flight, leading to a then-unknown FADEC failure mode, which was reason enough for the pilot to make a precautionary landing at an airfield.