In a serial hybrid configuration, all that is required of the generator's power source (say Wankel engine) is two states.  Off and On at MAXIMUM efficiency. Depending on electricity storage capacity (say Li-Ion) even spool times can be accommodated (turbine, maybe Wankel).  If a single rotational velocity with a single load is necessary to achieve the max, so be it. That is what low cost intelligent and power electronics are for.

The knock on serial hybrid is the inefficiency of two conversions at high speed - liquid fuel to electricity plus electricity to wheel torque. Toyota Hybrid Synergy Drive addresses this with the power split device.