This! Anything unproven makes the flight certification process longer. They already have to get their electric side certified, might as well make the backup generator as easily certified as possible.
My father, a mechanical engineer, was a tireless advocate of the hypocycloid crankshaft mechanism. In the 90s, hybrid vehicles with a hypocycloid-based generator feeding the batteries that make the electric motors go was the application he was going for, especially since hypocycloids function best at constant RPM.