The only proven configurations that can withstand loss of a motor in flight are the flat octo, and the coaxial Y6 and X8. On a standard flat hex in certain circumstances it may be possible to land successfully on loss of a motor but that assumes a light enough weight that 5 motors can sustain flight along with a very proficient pilot at the controls. In a best case scenario a flat hex will still fly but more than likely lose most if not all yaw control so you wind up in a situation similar to loss of the tail rotor on an RC heli. Problem here is you can't just hit throttle hold and auto-rotate down to the ground like you can with the heli, you will have to fly the multi down while its rotating around the Z axis, possible but certainly not easy and more of a controlled crash than a landing.
Ken
