Other Advantages
A FADEC will deliver more power on takeoff than just manually setting at full rich. Full rich is NOT full power.
Also a FADEC system provides a lot of diagnostic data and reams of digital data. We once told a customer which plug, bottom cylinder 4 was loose. Someday warranty will be based on FADEC history data.
And yes a great pilot who understands his engine can run it just as well, while doing a thousand other cockpit task, but those pilots are few and far between. Exceptions taken for this forums readers.
A FADEC system will help prevent over heating by enriching the mixture to a cylinder that is overheating. The TCM system could adjust the mixture for each cylinder independently, a different mixture for each cylinder giving a very smooth running engine.