One of my senior projects in college was designing a ducted lift fan UAV. One of the things that was apparent from digging through the old research was the monstrous total complexity even of conceptually simple designs that had 1-2 ducted fans embedded in the body or wings. Computers make the complexities of controlling these things tractable, which was a much bigger problem in the 1960s. But even with relatively modern technology e.g. Lockheed has had a hell of a time with the F-35B.
When near the ground and moving slowly the wings are stalled and not providing any lift; they're essentially just dead weight. A bigger risk near the ground with older turbine engine VTOL designs was ingesting hot exhaust gasses back into the intake, thus causing a sudden loss of thrust. The F-35B mostly resolved that by using a shaft driven lift fan in the forward fuselage.
