Self Defence is different from competition fighting in the ring etc. For a number of reasons.
The main reason being ring fights have rules.
Even all the UFC, MMA, Cage fighting etc, they all have a huge list of rules, things that you're not allowed to do because they're too dangerous. So the only way to become proficient at this type of fighting, is to train the methods that are allowed, thus making them become your body's natural reflex action in a fight, and if that's your reflex, then that's what you'll use in every fight. So in the real fight on the street, you've there for trained yourself to not use the most dangerous strikes, and so your opponent will have the advantage, as he won't care about rules.
For example, if I was in the ring with a grappler style fighter, every method I know to defeat him is bannd! So I would be disqualified at first contact. But put the fight on the street, and it would be the other way round, the grappler would not be expecting the elbow to the back of his head, as he's use to playing where that's not allowed.
I've known Pro Thai Boxers that have been beaten up on the street because they have kicked the guy in the thigh and not the knee etc.
So in street fighting self-defence, we train in the most deadly methods possible, so that becomes our reflex! If it's some young kid with an ego trying to show off etc, then this is not a real fight, and you can still control your power and strikes so is not to kill him.
A real fight is when you're not expecting it, and where the guy is attacking you full force and trying to kill you! In this case if you use anything but the most deadly stuff, then you'll probably be beaten.
We aim at areas that can take just one strike to knock him out. We train to strike from very close range, and we train to keep striking until it's over. So many people ask, "but I don't want to go to prison?" Again we're not talking about some guy in the pub who thinks you looked at his girlfriend. We're talking about someone trying to kill you and your family, I'd prefer to know my family is safe at the risk of going to prison, rather then there being any risk of them being killed.
It's much easier to tone down your power if the fight is not so dangerous, than to not be ready for a real fight by training is safe methods.