Put another way: I was told by a friend of mine that many schools tend to teach techniques that will work on the average joe or against another person from that style, but don’t tend to incorporate ways to defend against a different style. Any truth to this?
Definitely. The question is what do you want to get out of these classes? Martial arts are a sport. It sport fighting, its not a self-defense course. If you are attacked while walking home youre a lot better off with a basic crotch kick or running away then putting up a dragon pose and attempting a flying kick.
Some people think mixed martial arts might be better, but at the end of the day this all assumes that there will be no dirty fighting, no friends of the assailant, no weapons, following of rules, etc. That’s a silly assumption.
That really depends on the style/system. Sport fighting styles, yes; combat systems, no.
There are styles like TKD or foil fencing that have been distilled from their combat origins to the extent that they are generally not useful outside of an arranged match with carefully observed rules, and that may be the sort of thing to which your friend refers.
I think that most successful combat schools teach what would be effective against that which their students perceive as common. When everyone you know practices a particular style of karate, you might be drawn toward a school that offers tactics tailored to counter that style. When the threat you perceive as most likely in your life is the pistol-wielding ambush attacker, you’ll likely be drawn to a school offering tactics to deal with that.
At a high level of serious and determined training, a practitioner of any combat system will be broadly effective against an individual hailing from any style. Once you’ve got down the basics of your style, you can then start adapting those fundamentals more broadly with increasingly challenging scenario training.
This is not quite true for sport-fighting styles. Boxers train pretty exclusively to fight other boxers, judoka train for other judoka, fencers for fencers, TKD for TKD, etc… This makes perfect sense because, as a sport fighter, one generally knows exactly what style he’ll be facing on the mat.