I've been ridding for the last three years, and I've honestly only completely fallen down 2 or 3 times - and that's because I was trying to do something crazy like wheel walk. I've never hit my head.
For some reason people think of it as being unstable. Granted, you're on one wheel, but it's just like walking as far as being upright goes, and it's not like you're stuck on it like you are on a bike.