Energy density combined with mechanical efficiency. I could see something like this being viable if they could find a way to make it a hyper-efficient VTOL fixe

Energy density combined with mechanical efficiency. I could see something like this being viable if they could find a way to make it a hyper-efficient VTOL fixed-wing craft, but quad/hex/octo-copters from an efficiency standpoint are really quite poor electricity-to-thrust converters.