Some techniques are inherently difficult to control, even among experts. High-target chamberless kicks such as crescent and axe kicks are nearly impossible to pull. Crescent kicks are okay if you can clear your opponents head, showing that you could have made contact. Not so with the gravity-charged axe kick, which implicitly targets the face or collarbone. Bear in mind that it only takes roughly 12 lbs. of pressure to break the clavicle. Also, overreaching with the axe kick means you'll end up striking with your Achilles tendon, not the sole or heel of the foot.
Anything that involves a full-body rotation to launch your move, such as a spinning backfist or any variant of a spin-around kick are specifically designed to increase power via centrifugal force. Think of a shot-putter torquing his body before he explodes that 16 lb. steel ball off his shoulder. If you see someone KO'd at a tournament it is quite often from a spin-around back kick, the most powerful kick in karate. And those things tend to come in hard, fast and wild, capable of breaking ribs, teeth, or anything that gets in its way. A few years back two BBs were sparring at our school and one of them got nailed south of the border with a spin-around back kick. Because this genius left his groin protector home that day he was rewarded with a trip to the hospital and emergency surgery. Some lessons are learned the hard way. Protective gear has its place, but too much of a good thing can be a problem.
When you're working self-defense with a partner remember that you're employing time-honored methods such as joint-locks and pressure-point applications designed to impart extreme pain. Play nice. And this goes both ways. If you're a student looking to one-up a senior or instructor you may be in for a really bad time.