I actually often hear the opposite complaint: that so many of the unschoolers they hear about are über-achievers, that it's hard to get a feel for what mere mortal unschooling kids are like.
My kids are pretty "successful" by most measures of achievement. They've all chosen to attend high school, mostly part-time, as teens, because we don't have a community college here and the high school offers a similar college-like set-up. Nevertheless the bulk of their learning has continued to thus far be outside of school. When they've attended school they've mostly taken courses a year or two or three ahead of their age-grade. My eldest (17) is currently doing a gap year of intensive music training in a big city (living on her own: gack!) and has been assured by the head of the department and two of the audition committee members that she will be a shoe-in for admission to the college of her choice and a serious contender for a full-ride scholarship when she applies to the Bachelor of Performance program this year. In her senior high school courses she was the top student in her (very small) high school and scored in the top 1% across our province. She reads cosmology and philosophy for fun.
At the other end of the age spectrum, my 8-year-old shows all the signs of high achievement too. She's reading exceptionally well, enjoys learning from materials intended for children 3 to 4 years older than she, is an advanced violinist, loves basic algebra and is handy with multiplication and division of decimals and fractions.
So yes, unschooling can and typically does an excellent job of preparing kids for success in life. Watch that barely-literate 12-year-old over the next few years and you might be pleasantly surprised: many unschooled kids develop a real penchant for academic skills once they become teens and quickly "catch up."