I do think it depends on the child's life 'in the round', as well as their specific character.
DD dances - a pressurised, competitive, body-conscious, appearance-focused, perfectionist, very female-dominated world (her dance school is very good about the body consciousness thing, but the fact remains that she spends 12 hours a week in leotard and tights in rooms lined with mirrors ).
2 all-girls options were available to her for secondary, as well as a mixed school. We sent her to the mixed one - despite the dancing, she has always got on better with boys, had many male friendships and relishes the often much more straightforward social interactions that boys have rather than the more complex friendship games played by girls.
Had she had a strongly mixed-sex out-of-school life - for example, she used to be involved in Scouting, she could like DS be involved in music, but both these have been overtaken by dancing due to sheer pressure on time - then we would have considered the all-girls option more seriously, because it would just have been a different way of creating balance. But dance every night and all-girls during the day would have been too much for her, given her particular character.
I do think it also depends on the presence or absence of boys in the family circle. I was at an all-girls school (chosen because of its academic excellence and 100% scholarships, and because the era of mixed-sex boarding schools had barely dawned at that point) but had 2 brothers and a range of male cousins. Those who found things more difficult - and at the time there were a fair number of highly stereotypical 'move to the newly mixed sixth forms of all boys schools and go off the rails' type events - tended to be those who had fewer male influences elsewhere in their lives - in a way that i think would be much rarer nowadays.