But it shouldn't be that hard to set up old-fashioned carburetion and ignition for maximum efficiency in one condition -- cruise flight. Once you have that, how much more will a bunch of silicon add?
A lot more reliability for one thing, and better starting and power delivery in the take-off and climb for another. ECUs optimise the ignition timing and the charge in the cylinders hundreds of times a second, and the engine therefore runs at peak efficiency and reliability at all power settings and atmospheric conditions.
Carburettors are very crude mechanical devices and are prone to icing and malfunctioning. Can any piston aircraft manually adjust their ignition timing whilst airborne?, I've not heard of that, but have only flown SEP and MEP Cessnas and Pipers etc
Of course, efficiency is of little concern to the SEP and MEP world, and a simple, large displacement magneto engine will run with no external electrical power, which has a certain advantage.
It was similar in the car world, where we used to have large capacity engines to ensure sufficient power in all conditions. Nowadays with electronics we have engines half or a third of those capacities but which are just as powerful, and more reliable and cleaner. In addition they will adjust themselves to prevent pre-ignition (knock) and will not allow themselves to be overrevved etc.