Calculating kW of rotational power required by an engine to generate 3 phase power when phases are unbalanced
Consider the case of a single-phase, 2-pole generator; the load would vary with the angle reaching a peak every 180° and zero in between those positions.
With three phases the loading on the engine will be smoother even with some inbalance.
Add in a flywheel and whatever other inertia is there including the engine and alternator rotor and I'd say you can average it out for modest imbalance.
You might find my answer to https://electronics.stackexchange.com/a/390400/73158 of some interest.