Generally, ducted propellers are more efficient. And yes, these blades are effectively rotating knives. Even small drone motors and propellers are very dangero

Generally, ducted propellers are more efficient. And yes, these blades are effectively rotating knives. Even small drone motors and propellers are very dangerous as the motors have a lot of power and high RPM, and the more high-end ones are made of CFK and have sharp edges. As these four do not only lift a drone, but a whole robot frame, I'd assume they'd easily cut through a finger.