The advantages of a single blade over the equivalent 4 smaller blades is efficiency. A tiny part of that is losses from extra motors but mainly its from blade l

The advantages of a single blade over the equivalent 4 smaller blades is efficiency. A tiny part of that is losses from extra motors but mainly its from blade length which dramatically affects power efficiency, up until the point where the tips go over the speed of sound, helicopters heavily optimise for this. The duct itself offers some advantage but it's used here to vector thrust for control rather than just go up and down. Battery power is quite constraining already for aircraft, ducted fan offers some benefit. The US military use a petrol powered ducted fan drone operationally, though i believe that's dual rotor, this is single rotor that offsets counter rotation with vectoring.