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Not if you're careful.
If the bike is dry when you take it out, and its -35C out, there isnt much moisture around to freeze into the cables.
Either will work fine. Especially With a low temp oil. The reason the Alasken crew run cable discs, is that fixing a fault in an hydraulic brake can be near impossible on the go. And at -35C you dont want to get stranded for long faffing around in the middle of the Alasken outback.
Silly question perhaps, but are hydraulics ok in serious sub zero temperatures like -35?