yeah jkd low kicks come in alot of forms, the lead side kick can either be a snappy jab with little body in it, a stop hit, or a power shot (like a mid body side kick) or any combination.
I have seen someone's shin break, not in MA class, on a soccer field, the cleats went through the guard and broke his tibia, I don't know if his fibula broke, he didn't get up. I have had all kinds of leg injuries but I realy don't know if I ever had a fracture because I have never had any part of my body except my head xrayed, but I do know people who played soccer on a fractured shin, the just thought they had a realy bad bruis, as for kicking hard enough, unless you crack the bone to the point that its structural integraty is in question when weight is put on it, you anre't gona take them out of a fight. And yes I know this is repeated info, but I don't realy know how else to put it, I have played soccer with people (ok one person) who was playing on a cracked shin, I myself have never gone to the hospital for such an injury, so I would say I have never been kicked hard enough, but unless someone broke my shin I don't see how that would be hard enough...
As to brekaing the shin, to break the shin your opponant has to have their weight on it, otherwise their foot will move, the only times I know of shins breaking are either dual impact (they are both kicking) or all their weight is on one foot (like running). if you need to break someone's shin I would suggest a side kick with the heal of the foot. As to breaking the knee, you don't break it, it's a joint and depending on it's position it is realy hard to dammage it, if someone keeps their knees straight than yeah you are gona hyper extend it, if someone has their knee bent a little and twists their leg a little side to side, you will just buckle it.
KennethKu breaking boards doesn't realy mean much beacause wood and bone are two very dissimular substances, in addition you rarely will kick a target that is imobile (pinned against the ground or a wall for example, and in any other situation the closest comparison would be a break out of the air.
as to the statement you made of most martial artists you know.. Well that has no bearing on gous statement, most of the martial artists I know practice JKD, does that mean most of the martial artists in the world practice JKD? obviously not. no one ever said that the people you know and are talking about can't take a boxer, we don't know them so it isn't even an issue, your personal freinds have no bearing on the statement in question though.
As to the TKD fighters kicking low? most TKD practitioners do not practice kicking low, don't know much about karate but from the people I know that used to practice it, I gather that kicking to the shin wasn't as big of a deal as kicking to the knee or the ribs, but I don't even know what style of karate it was, not the greatest example

. My only real point so far has been that not all martial artists are your freinds, or rather your freinds aren't representative of most martial artists.