My children are homeschooled, but they will attend public school next year (entering grade 6 and 9).
I think they both have a good sense of self at this point, and I think they will get out of school what they want to get out of school. In other words, they won't be caught up in peer pressure, and silly manipulations, because they already love their life as it is.
So, to me, the biggest disappointment I would have, if my kids attended school all along, would be the inability to think for themselves. Sadly, for kids, education and academics come after feeling accepted by their peers, and kids often do things that don't feel right. This is such a big distraction, that academics fade in the background, and what they remember is the emotional experience of the day.
Success or failure are all about the child reaching their goals - and the unintended failure of the public school experience, is that they aren't setting their own goals, they are often trying to fit into someone else's goals.