S&W had problems with their first generation Sigma, but it occurred due to shrinkage and not brittleness. In addition, the thermometer hovered around -10* when the problem presented itself.
I do remember field reports from the boys up north of synthetics, not necessarily plastics, having brittleness issues in extreme cold temperatures. I don't recall what the temp range was when the problems arose. This was in the early to mid '80's.
I don't think I'd worry too much about plastics today. Enough time has elapsed that I think they have evolved enough to eliminate any issues and the engineers have learned about the cold weather characteristics.