Being fixed pitch fans, I wonder how the system will perform during autorotation where it will have to produce both positive and negative thrust in order to control yaw? I would imagine the negative thrust scenario might have a different subjective feel to the pilot because of inefficiencies associated with the fans running backwards.
Also, the system might feel a bit laggy as the fans change rpm. If any of you fly RC helis, the difference in response is night and day between a fixed-pitch rotor and a rotor with collective. Having 4 smaller fans, compared to one larger fan, might mitigate this issue a bit, however, because of the ability of the smaller fans to respond more quickly.