This actually isn't true, about kids struggling. Unschooling kids adapt as well as their natural aptitudes allow - as well as or better than school kids with similar aptitudes. It seems counter-intuitive, but it's a good example of how teaching doesn't guarantee learning. Being "in the system" doesn't necessarily make it easier to handle the system. Sometimes it makes it harder, because kids burn out academically, or get overwhelmed socially, or get so stressed they struggle to function.
Unschooling kids are used to making choices about things that matter to them. It doesn't mean all their choices are good ones, but by the time they're young adults they've had a chance to learn that, to learn that sometimes you fail and move on. Sometimes you get stuck and need help. Sometimes things don't work out the way you want and you regroup. And because they haven't been pressured to succeed, they're better equipped to deal with those possibilities. That's an asset in higher education.
They're also used to persevering when things matter to them. That's something else that can seem really counter-intuitive to people who aren't used to natural learning (which includes almost all unschooling parents, at first!). Schooling presumes that kids need to be taught to persevere - but kids are actually really good at it when something is important to them. Ironically, the kinds of things that motivate kids to persevere are also the things parents tend to wish kids wouldn't do. Kids will struggle with a project until they're crying and swearing and throwing materials and Not Want to Give Up. And parents respond with "stop that nonsense and go play." And then wring our hands at how poorly motivate our students are.
Given the chance, kids are actually great at learning the skills they'll need in life. Even the weirdly artificial kinds of contemporary life we've built for ourselves. Sure, they need help and support, but they don't need lessons on how to be better humans. They're darned good humans from the start .