A properly executed roundhouse kick can be devastating. It's hard to anticipate because it's coming from the side and often doesn't enter the opponent's field of vision until too late. For the same reason, it's also difficult to block. This is the common knockout kick in mixed martial arts competitions like the UFC.
There's a fair amount of power behind the kick because of hip and body rotation. And because it's coming from the side, it's one of the more difficult kicks for the opponent to catch and throw.
The downside risk is injury to the ankle or knee of the attacker if performed improperly. There can be injury to the kicker's striking ankle. If the attacker uses the ball of the foot as the striking surface, it's fairly safe, but also harder to be accurate with the strike. If the top of the foot is used to the head, the ankle can become hyper-extended and sprained. Often attackers try to use the lower part of the shin to hit the opponents head or front leg, and that's effective. But if the opponent moves backward, the top end of the foot could become the striking surface, unintentionally putting undue stress on the ankle.
There is also potential of injury of the kicker's standing knee depending on ground surface or footwear. Knee injury can occur during the rotation if the standing foot sticks to the ground. Practitioners often kick off the ball or heel of the standing leg rather than let the foot remain planted to prevent this kind of rotational injury. This is really important if wearing shoes. Rotational injury of the knee could result in ligament sprain or even tear.
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