I've used my fatbike in -20. Brakes (Avid Guides) were fine. I have never used them in -30. In cold weather go with Avid or Sram. Dot fluid is supposedly better in the cold than mineral oil (Shimano). Mechanical is most reliable in the cold.
Found a link
https://www.velonews.com/2015/01/bik...weather_358172