If the aircraft is maintained on LAMS it will still have a 50hr check regardless of the engine requirements. It may need servicing every 100hrs but at least on a Lycoming you don't have to change the crank balance dampers, strip the fuel pump down, change the cambelt...
The PA29 conversion with the Centurion (was 1.7 Merc, now their own 2.0 supposedly), weighs about 120lb more, cruises 5kts slower and has a poorer take-off and climb to 2000ft than a standard Lycoming version. Oh, and all with the help of a CS prop. What an excellent mod. Not....
Fuel costs are only one small part of the cost of running an aircraft. Perhaps when the Americans use diesel in everyday life rather than just huge trucks then the market might open up - oh and perhaps if the reliability improves, that might help to.
Not just Centurion. Allegedly there was the case of a 30p resistor failing in a SMA powered C182 FADEC recently which put the engine at max chat on start-up. A van stopped it eventually (aircraft beyond economic repair apparently) and lucky the hangar doors had just been closed before it went in there. Bit embarrassing, it being the demonstrator....