My understanding is that, other than displays, most electronics are
rated from -40C to +125C. (Displays have special issues with moisture.)
I don't think that it's your speed controller is freezing up.
A more likely cause for your woes is condensation in the
connectors. Since water expands when it freezes, it
could push the connectors apart.
You could confirm this by wrapping the connectors with electric heat tape
(like you put on your pipes).
Good luck and stay warm.