I agree with some of the positives that others are posting re the female being the default, but have a few cons to add.
I went to a mixed primary school and girls only secondary (in the 00s). My girls' school was very bitchy and cliquey, but maybe a mixed school would have been the same, or maybe it was that I had a particularly bad year group. I have to say that I don't remember any kind of "girls supporting girls" energy. It was very competitive. Could have just been the school, though.
The biggest thing I remember was that access to boys was the most valuable currency you could have. Past Year 8, the biggest thing that made you cool was knowing boys.
I didn't have brothers, male cousins, or male friends from extracurriculars, so for years, I didn't speak to a male peer. I was shy but averagely so, not ND or anything - I just didn't know any boys. My equally-uncool friends were in the same boat. The more popular girls would go off with groups of boys from nearby boys' schools after school, to sit in parks and drink, but if you weren't in with them, that was kind of it - you just didn't speak to boys. I remember being 15 and desperate to have a first kiss and a boyfriend, but with no idea of how to make it happen. I sometimes felt that I was living a 1950s teenage experience.
When I started university, I was very nervous around boys. I was hugely naive about what actual men are actually like, mostly because I had spent more time reading romance novels than interacting with actual males. I developed huge, debilitating crushes and, I think, behaved in a very immature way for an 18 year old. I think university would have been a bit easier (and also more fun!) if I had got some of that out of the way at 14 rather than 18. I didn't have a boyfriend until I was 20.
So that's my view - for some of the girls I was at school with, it was fine, and it might have been fine for me if I'd been friends with the right people, but for some of us, we were effectively cloistered, and it seemed to come down to where you were in the coolness pecking order. It's easy in middle age to dismiss it, but romance it's a big deal to teenagers and a big part of your development.
Anyway, this is all quite personal and may have been specific to circumstances, but wanted to add it as I think there's sometimes a starry-eyed view of female single-sex schooling on here. If I HAD to send my kids to single sex schools, I would try and make sure that they had some extracurriculars with the opposite sex.