The older 12 valve, 6 cylinder Cummings engines found in Dodge pick up trucks used a mechanical fuel injection system. Proven, the problems are known and easy to fix/prevent, and lots of power can be extracted if you don't care about clouds of smoke. Only one wire to connect to the ignition switch, and another to the starter. So, yes, full authority analog engine control, with One Lever, the throttle, is easy to make, relatively. Poor choice, as is for an airplane, as it masses over 800 pounds.I think if there was a significant demand for long range, slow light aircraft, we'd see more diesels developed. Wonder if it's possible to have some kind of mechanical backup for FADEC, or a system that really doesn't need much of a battery when the alternator is working.
However, the reason everyone uses computers today, is pollution. Control of the combustion event, fuel injection timing and amount, gives better mileage, and cleaner air.