Well, at the risk of throwing water on a hot topic, SYG can be a bit of a problem, and we managed to do pretty good with self defense for a long time without SYG. SYG basically does away with the traditional preclusion test that historically has been a part of analyzing self defense.
Look at it this way: I go outside to get in my car. My neighbor starts yelling at me about my cats using his rose garden for a litter box. I sort of blow him off. He yells he is going to beat my brains out and comes toward me with a shovel across 40 yards of open space. I can get in the car and leave. I can go back in the house and call the police. But instead I go to the edge of my driveway, get a firm grip on my CCW, and when he is about 21' away I draw and shoot him.
Under traditional self defense I would need to show why it was reasonable and necessary for me to do what I did. Under SYG I just have to show that I was in reasonable fear of death or great bodily injury. Under SYG since I was where I was legally and what I was doing was legal I had no obligation to even try to come up with a lesser alternative or avoid the shooting.
I support SYG but it does create a new legal concept, essentially taking Castle Doctrine out of the castle and into the street. As I tend to support "shoot when you must" instead of "shoot when you can" I do see some issues with it, and that is why it is controversial.