Hmmm... hardest engine to maintain? I'd vote for EFi engines... too many things plonk under the hood. Unlike the good ol mechanical carbie days... and contact point (old type distributor).
EFi engines are better in fuel consumption and tier-to-tier performance upgrade... ie: first level : mechanical upgrade
air filter, air ram, exhaust system, porting, etc...
second level : electronics
ECU, piggybacks, digital add-ons (SAFC, RSM, Profec...etc)
- the more stuff added, the harder to detect who is the culprit when engine kaput.
Mechanical engine has lesser stuff... the essential of mechanical engines : carburettor / contactpoint or distributor and ignition coil... then comes the head and block, gearbox, alternator... easier to detect fault. Even a 4 throttle Dellorto or Weber (rare indeed) is easier to maintain than EFi system.