Age is a really important factor.
DH boarded from 7 - Year 3. He 'liked' it, in the main, at prep school, but very much disliked senior school. His relationship with his parents was and is that of a polite stranger - he describes it as being like a lodger in his parents' house. It perhaps didn't help that he has a sister who didn't board, whose relationship with her parents was extremely close and loving, so he (as anyway the quieter character of the two) felt marginalised.
I boarded from 11 - Year 8 started a year young. I was just about mature enough to understand the compromises being made - a truly amazing education obtained at the cost of having to board. Because my parents took the time, and I was old enough to appreciate, the reasons behind the decision, my relationship with my parents remained very much intact.
Attitude also helped. DH was always told how lucky he was, the sacrifices being made for him to go (even now, 35 years on, it is still the first thing DH's parents talk about when they mention him - all those sacrifices) etc, and so never felt able to say 'actually, I hate it'. My parents were very clear that if I was unhappy, I could leave.