I'm really surprised by how little attention this key fact is getting. The best engine for a range extender duty is very different from the best engine for direct drive, internal combustion or not.
but is it worth the extra weight and complexity of a second engine and fuel source?
The answer at present appears to be "yes, unless your use case allows frequent recharging" (such as an EV commuter car/grocery getter, alternate transportation available for long trips, no likely situation where unplanned long trip would be necessary) or there's large financial incentives to ditch the engine (i.e. taxpayers paying people for the inconvenience).