Unschooling is entirely child directed, with very little to no structure or educational requirements.
It is NOT the same as eclectic homeschooling, which uses many different methods for homeschooling, including (but not necessarily requiring) published curricula, unit studies, unschooling, on line schooling, group classes, etc.
I have known a few unschoolers who got an excellent education and showed no deficits academically, behaviorally, or socially.
I must admit though, that most of the unschooled children that I have met are far behind in basic subjects, many bordering on ignorant, and are very poorly behaved, so much so that if I am leading an activity I cringe to see them show up.
In a homeschooling group unschoolers often stick out because most other homeschooled kids are academically advanced, socially mature, polite, and well behaved.
I think for some children and families, unschooling works really well, but parents really need to pay attention to whether it really works for their children, and switch homeschooling philosophies/methods if necessary.