Advantages of ducted fans
For almost all aircraft designers, their drawbacks have weighed more heavily:
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The highest propulsion efficiency is possible with a large, slow-spinning prop. Once flight speeds approach the speed of sound, the propeller has to become smaller to avoid supersonic propeller tips, and then the shrouded design becomes more attractive. Electric airplanes so far are not fast enough to profit from this effect. I would be very skeptical of the increased thrust claim – both efficiency and thrust of the E-Fan ducted fans are lower than what a well-designed propeller can offer.