What are technical and aerodynamical limitations to blown wings main propulsion? Why wouldn't some blown flap or blown wing device (suppressing propeller or fan or jet exhaust from aircraft design) produce sufficient thrust needed for takeoff and cruise flight, opposed to "only" providing stol capabilities at low speed?
In a nutshell, it's the inefficiency of expelling air out a long skinny nozzle which results in massive energy losses due to friction.  In other words, too much duct surface area relative to the amount of total air volume.