I think there's two components to this.
The first is training styles that don't really translate to the real world. Some arts may be more susceptible to this than others, but things like:
- Not sparring at all
- Sparring with very specific rules
- Too much focus on conceptual drills and not enough practical drills
Most arts are very good at fighting against their own art. Someone who is only trained at Wing Chun is going to be very good at fighting against someone who is fighting the Wing Chun way, but since nobody else fights like that, their drills may not be the most practical. Someone who is trained in modern Taekwondo may have great kicks, but if they don't know how to use them against a puncher or a grappler, they're toast. Grappling arts are the same way. A BJJ expert is going to be better than just about anyone on the ground. But take a fight on hardwood or concrete, and they'll be a lot less eager to use the majority of their training.
This isn't true of all schools, or even all fighters within a school. Someone taking Wing Chun may go to a school that teaches all of the aspects of the art, instead of just Chi Sau. A Taekwondo fighter can learn a lot about footwork and managing range, and still has very powerful kicks. They may go to a school that teaches a lot more of the martial arts than most of the modernized ones. And a BJJ fighter may learn a lot of the stand-up in addition to the ground game.
The second component is who you expect to fight. Let's go back to BJJ. How much of their skills are only necessary against another BJJ expert? If the average blue belt can beat the average white belt, then what do you need beyond that unless you're fighting against other highly trained grapplers? It makes sense for the sport, because that's what you're going to face. But I doubt the average person you need to defend against is a seasoned martial artist. A seasoned martial artist is much more likely to avoid a fight (unless it's a sanctioned bout). They're more likely to de-escalate or walk away from a situation than to instigate one. That's not to say it doesn't happen. Just that I think it leans heavily in favor of fighting against people of much lower skill level when you need to defend yourself.
This is important, because a lot of the arguments I see about an art being bad are because "that wouldn't work against a high-level ______." There seems to be the assumption that in order to defend yourself, the necessary skill level is:
- Defend punches from the heavyweight boxing champion of the world
- Defend kicks from a gold-medal Olympic Taekwondo fighter
- Out-clinch the top-ranked Muay Thai fighter in the world
- Defend throws against a Judoka Olympian
- Defend takedowns against a collegiate superstar
- Survive the ground against a BJJ black belt
This is not much of an exaggeration from some of the posts I've seen. I think there's an absurd standard that every martial artist needs to have skill levels and fitness level of a professional MMA fighter or Olympic competitor in order to be considered good, and that if your school isn't pumping out notorious knockout kings then you're school (and possibly your art) are completely useless. I don't think that's the case. I think the average person you're going to fight in self defense is probably:
- Barely trained in martial arts, may have some practical experience if they've been in a lot of fights
- Looking for an easy opportunity, and will give up when they realize you know how to fight (if you didn't de-escalate the situation)
I may not have the take-down defense to deal with a ranking wrestler, the boxing skills to go up against a proficient pugilist, or the ground-fighting skills to go up against a submission savant. I do think I have the skills to go against the average Joe who thinks he knows how to fight.
With that said, in the last several years that I've been in martial arts, I've gotten in 0 fights. In several years before that, I got in 0 fights. You have to go back to high school to when I watched my friend get in a fight...it was over in 1 punch. Not a KO, but the other guy decided it wasn't worth it. You have to go back to middle school for the last time I was in a fight. One throw (not even hard) and the other guy decided to keep making fun of me...but he stopped pushing me.
TL;DR: I think some people have too high a standard for what qualifies as legitimate self-defense.