I guess the biggest mistake I see is in assuming that radical unschooling, which I see as no specific educational direction provided for a child whatsoever, will somehow result in well-rounded people. It is beyond dispute that people will concentrate on what they like best, and many people do this to the exclusion of much else. Throw in a touch of the narrow-focused flavors of giftedness, and things may get much worse.
There are many instances of kids failed by school systems, but of course there are many, many more kids in the school system than are unschooled. Cherry-picking anecdotes about kids failed by schools doesn't lend merit to the idea that someone will be better-rounded left to their own devices.
If anything, it shows that despite stumbling attempts to impose some learning structure, some kids don't learn certain things easily-- and these are the same kids who would be most at risk with a radical unschooling approach: where even if a child is having a serious learning problem in a certain area, his parents are happy to have him otherwise engaged, as long as he seems happy.
I suspect this becomes more of a moot point as things go on, with more rigorous curriculum and testing requirements for homeschoolers in different states. As long as kids learn everything they need to learn at a minimum, and they're encouraged enough in their growth, I'm fine with any approach.
Anecdotes about some excellent radical unschoolers doesn't change the basic fact that kids aren't typically well-rounded in their interests, just as most wouldn't choose to add brussels sprouts into their eating rotation if left to their own devices.
