As someone who has trained Jiu-Jitsu for over 15 years, as well as wrestled in High School, and competed in MMA - I believe this original comment does have some truth, but also not exactly reality either. The real question I think is: Is Jiu-Jitsu effective against a skilled opponent in a violent street altercation.
There are a few thoughts / remarks on Jiu-Jitsu for Self-Defense:
Scenario #1:
If you're in a pure 1 vs 1 fight, and your opponent has Jiu-Jitsu and you have.... a ____ Karate style - my money is on the Jiu-Jitsu guy. The reason being is that the Jiu-Jitsu guy will know how to close the distance and bring the fighter to the ground.
Just watch some Gracie Challenge footage and you'll see:
Gracie Challenge Archives - Warrior Combatives Academy
OR just watch ANY UFC for the 1990s. The early 2000s were all about Wrestlers.
Scenario #2:
If you're in a 1 vs 1 fight, and your opponent has Jiu-Jitsu - but you also train _____ Karate, but spar / train in a more realistic standpoint to defend against takedowns, good in the clinch, etc.... then I'd say it's a 50/50 shot.
Scenario #3:
If you're in a 1 vs 1 fight, and your opponent has Jiu-Jitsu and you have trained in _____, but your buddy decided to jump in.... Jiu-Jitsu loses.
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I tell this story to my students all the time. I used to work Security at a popular night club for 9 years (along with bartending). A fight broke out one night and as I got involved, I ended up locking up with one fighter in attempt to bring him out. I don't remember the exact circumstances of how I got there, but I ended up on my back in guard. Cool right? I can now use Jiu-Jitsu to submit this guy and get him out. WRONG. I got kicked, punched, kicked some more, and held on for dear life hoping some other bouncer would come to help. Eventually they did. But my ribs, my head, my face... all paid the price. Jiu-Jitsu was NOT great in a crowd environment.
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Another story from working, as we were bringing people out, we all got shoved out into the street and the fight continued. I got pulled down from behind and then another "opponent" started attacking me from the front. I was able to use tome nage (to a certain degree) to throw him off and then use the technical stand-up to get back to my feet. Jiu-Jitsu worked then because I was able to use it to get back to me feet and fight from there.
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Final Remarks:
I personally believe (and I am biased) that Brazilian Jiu-Jitsu is the ultimate ground martial art. And the REASON is not just because the techniques work, but because the way BJJ is trained. BJJ fighters spar (roll) hard EVERY class. 45 minutes to an hour a night - we roll. That's typically 50% of the students training is live fighting. You can't even begin to understand the benefits of what the students are learning in those sessions. Not only are they developing their skills, but they're also developing sensitivity, awareness, and the "feel" of a fight. Irreplaceable.
If more styles employed more realistic fighting / sparring - I believe they would be more effective in all scenarios.