What makes Mariner think that GA electrical systems are unreliable? In the present designs they are very considerably more reliable than the recip and rotating elements. Then add in the fuel, carb icing and the nut behind the wheel and the element of risk from the electrical system failing is so tiny that it is negilible.
When Mooney put the Porsche engine in their aircraft it had a conventional (car type) ignition system and dual generators, buses and batteries - largely to meet the certification rules at the time. That made it complicated and heavy - not good things for an aircraft. The trick is KISS, two engines actually gives you more than double the chance of an engine failure - and most light twins simply delay the forced landing by a small period.
It is a little surprising that the FADEC does not have a 'limp home' ability - but you have to loose the generator and the battery to loose the FADEC - which will doubtless warn you of low voltage etc. long before the engine is likely to give up. A dry cell could probably keep the unit running for a couple of hours you're really worried about it (but of course you could never get that option certified!).
Given the numbers for engine failure from the present fleet, concerns over electrical systems in diesels are simply not warranted.