There are several methods of executing the Spinning Heel Kick, some more effective than others, but all capable of doing damage to the opponent. There are many objections to it for tactical or personal style reasons: The most common criticisms are: The Spinning Heel Kick is most often aimed high as a Head Kick, and consequently it is easily dodged by experienced fighters. Others object to it because it consumes a great deal of energy and the kick requires turning your back to the opponent which telegraphs the intent. Finally, some object on the grounds that it is a "fancy or exhibitionist" kick. But in this last objection I have never found anyone who makes this criticism that could actually execute a Spin Kick, and leaves me to believe that "fancy" means they can't do it.
Early affiliation between Hapkido, Kuk Sool Won and Hwa Rang, Circa Mid-1960's shows connections between the arts.
Hapkido was derived from Daito-ryu-Aikijutsu, this page contains excellent history information.