Yes, there is nylon at the shoulders. I guess I never really noticed if my mare sweat in it or not. I want to say no though and the days were really hot too(around 100 degrees all the time). But, really, I don't think so. Then again, it might depend on the horse.
The weatherbeeta airflow mesh made my mare sweat though. I remember seeming the white ''foam" on her shoulders and back when I took the sheet off. But then again, the sheet was too small for her
I guess the only disadvantage I had with the amigo sheet is that it does have a lot of straps that you have to do when you put the sheet on. I'm mostly talking about the belly band. It is meant 'not to come undone', which is a good thing. But all in all, I found it to be nicely designed, hardy, and well made. But people did tend to make fun of my mare.
They called her the 'ghost' horse! HEHE...but then again, with her mask on and everything...she did look really funny!!
I bought my sheet at smartpak.com. It was around $130-135.