[Geebee]

A series hybrid that only uses the motor say 8% of the time its running is a hell of a lot more efficient than say a current Prius.
The genset for a series hybrid does not have to be capable of the entire load as the batteries will buffer it, so it can be smaller, genset location is basically unlimited allowing more efficient design and can be optimised for a singel rpm at absolute max efficiency.
I am more of the opinion that a parallel hybrid is the dead end tech. you are better of just optimising the ICE.
Oh and I forgot the losses through the required transmision to match rpm to a usable ICE range