Hello Dave,
I have yet to see any substantiated evidence that serial hybrids are inherently less efficient.
I have offered three substantiated cases where serial hybrids are more efficient:
Diesel/electric trains -- proven because they are more efficient than anything else. They are almost 3.5X more efficient than trucks.
Mini Cooper serial hybrid prototype gets ~2X the FE of a standard Mini Cooper.
FVT alé gets 92mpg (maximum) while the FVT eVaro serial hybrid version of the same vehicle gets
165-275MPGe.