Are you talking about FADEC on a petrol engine? It appears closer on the diesels but they are far from reliable at present.
Are you talking about the deisel engine being unreliable, or the FADEC systems? If you are refering to the DA40TDi, I seem to remember reading that it depended upon who had been doing the maintenence.....
I also thought (maybe wrongly!) that if the FADEC electronics on them failed, the engine just kept on running (as per normal with a diesel engine), I remember seeing a demo where the electrics were all turned off on a DA40TDi and the engine just kept on running......
I admit to being a fan of the diesel/JetA1 engine, and hope that any problems there are can be quickly ironed out, because I sincerely believe that unless we do something to reduce the costs of light aircraft flying, it will die out completely.
Regards, SD..