I agree with DOM, speed and accuracy are key. In class I often have my kicks caught for two reasons: firstly my partners are longer trained and faster, and secondly my kicks are on the whole awkward and so slow they are telegraphed. I would never kick above the waist in a real fight I could do far more damage with other techniques. I am short and slight in frame, and not especially strong, but I am faster than many in my class. I would use my upper body if I really wanted to injure in a fight, using speed, accuracy from training over and over, and knowledge of vulnerable points. And these I think I would only use if backed up and cornered. Otherwise I would probably use a kick, but to the knee, ankle, or shin and designed to hopefully disable to buy me running time. Training in the classroom is one thing, as you say, you can set the rules such as no kicks to the groin. I use high kicks in class as the people I train with are skilled and controlled, getting hurt is an infrequent accident. I have the luxury of “instant replay” and discussion. But at home I practice snaps and scrapes, keeping low and fast, this is where muscle memory would hopefully serve me well. I think Yue that if you are quick, accurate, and confident in your upper body kick it will probably work in a real fight, especially against someone unskilled in martial arts. A thrusting kick to the chest, if fast and unexpected and you have good root would have your opponent on the ground, or bent over gasping, and gives you the option of taking it to the ground or buys you running time. I can think of at least three people in my class who could do as DOM describes, knock out with a kick to the head, snap, quick, before they even glimpsed it was coming. And they are all of wiry build, average height, ridiculously fast and strong. They also have years and years of training tho, and have used their martial arts.