Now we are talking about controller efficiencies?
If I had an EV that I wanted to use past it's EV capabilities I would hands down, no question go parallel and put that ICE power to the wheels. It has to do with:
1) the intended usage patterns and
2) it has to do with efficiency.
If I am doing anything within city limits or up to about 10 to 20 miles out from home, I'd want my EV to handle all that unassisted, and equip it so. And in fact my EV does (the Songi bike).
However when I finally venture out of my little world, I REALLY venture out and my trips are commonly 60 to 600 miles one way out of town. On such trips I'm not willing to sit around and recharge either the original battery pack or an auxilliary pack (so that makes an auxilliary battery trailer, which would be an excellent solution for the <60 mile trips out, a moot point); I go pretty much straight through and that demands ICE power in some form.
If I'm going 600 miles I don't want to use a genset in a serial fashion; fe is
guaranteed TO SUCK. 
Then I would be better off abandoning the whole EV concept (not dragging a heavy, costly, mostly disabled EV drivetrain and battery pack around for 1200 miles) and stick with an efficient ICE.
And that's pretty much it, for me.
Others who only ask for a wee bit of extra range, and only occasionally, have other choices: they could add a genset serially, or they could throw some more/better batteries on and stay true EV... or they could put ICE power to the ground too, as fooling around with all this Rube Goldburg equipment is ALL ABOUT THE EFFICIENCY... right?
YMMV.