What are the advantages of a Spinning Wing (Magnus Effect) and why haven't any been commercially produced?
Some earlier prototypes of spinning-wing aircraft were produced but none were successful. The design has some very serious disadvantages. One of the main issues appears to be undesirable gyroscopic effects.



Production


  ... why it was never produced?


Several full-scale prototype aircraft with rotating Magnus-effect wings seem to have been produced:


921-V - Plymouth. 1930.
X772N - Union Aircraft Co. 1931.


It may be that 921-V is the only one to have flown, crashing after one flight.

   The Flettner airplane  


  Built in 1930 (USA), the 921-V is reported to have been flown at least once - ending it's short career with a crash landing. Three cylinders with disks performing as winglets driven by a separate engine. Information on this design needed! It's probably the only aircraft equipped with cylinder wings which made it into the air.


From Pilotfriend.com

In practice the effect can be less efficient than conventional alternatives


  In the early 1920's the force from a rotating cylinder was used to power a sailing ship. The idea, proposed by Anton Flettner of Germany, was to replace the mast and cloth sails with a large cylinder rotated by an engine below deck. The idea worked, but the propulsion force generated was less than the motor would have generated if it had been connected to a standard marine propeller! 


From NASA



Advantages


  What were the advantages?


The article referenced suggests


  More wing lift and less drag are the major aims of aviation’s researchers. Maybe the Magnus Wing will supply the answers ... The ship is powered by a conventional gas-turbine jet engine, while the drums are revolved by a separate piston engine. Small wings, heavy loads and quick take-offs would be its great advantages.




Disadvantages

The developer of a working model wrote about a few of the disadvantages (try a google translation of that page for more):


If the cylinder rotation accidentally becomes slow or stops, its lift disappears completely. This plane will never be able to glide. 
If a gust blows from backside during a slow flight (takeoff or landing), the cylinder wings generate a down force. 
The spinning cylinder wings generate a strong gyro effect, which makes it difficult for the plane to change its attitude.


See video and comments.



NASA did some experiments with rotating-cylinders for flaps (not main lift source)



They concluded


  These experiments demonstrated on the one hand the
  effectiveness of such a high lift system but on the other hand the weakness in handling qualities due to the gyroscopic forces in such an aircraft configuration.


From A review of the Magnus effect in aeronautics



The US Army also carried out a study of the use of the Magnus effect in aircraft
VIEW AND PRELIMINARY EVALUATION OF LIFTING HORIZONTAL-AXIS ROTATING-WING AERONAUTICAL SYSTEMS



It is difficult to pick out a simple conclusion as the study looked at a wide variety of systems. For rotating cylinders in wing (RCIW) they wrote


  Such systems do not appear to have merit as STOL devices.