Basically, I was allowed to skip schoolwork, as long as I was doing something productive.
I hated schoolwork, so I started spending all my spare time tinkering with stuff on the computer. I wasn't allowed to play video games (since they aren't productive), but I was allowed to BUILD video games. I was also allowed to build web sites and do programming.
By the age of 13, I was doing contract web development work, and by the age of 16, I was employed full time at a local web development company.
My parents wouldn't let me get a drivers license and car until I had at least passed the GED, so I spent a couple of months cramming for it by studying a 2 inch thick book about how to pass the GED. I passed it without too much trouble (I think I got a 98% in math, if I remember correctly).
My career has developed across multiple companies over the last 20 years, and I'm currently making around $150K a year as a software engineer. I never did go to college.
They took "unschooling" even further with my younger brother. He didn't even learn how to read until he was around 10. He ended up having to cram for his GED to get a drivers license, too. He ultimately ended up getting a job as a waiter, then as bank teller, then put himself through college and got a degree. He now owns a house worth around $400K and has a career as a software engineer at the banking company that initially hired him as a teller.
TL;DR: "Unschooling" worked for my family because they combined it with at least a base of traditional education.