Not agreeing or disagreeing with anyone, but I actually think Blak Fox has a point. Kun himself risked his life when he first did it, and he used it with all his rage. When he later on studied all about it and what Sadow had to say about, I'm sure he understood perfectly well what caused that beam to be so powerful that it was a risk to himself, so instead he used it on a smaller scale "because he could control it".