I used whoopies all last winter, and they got soaked and frozen several times.
Water does infiltrate between the strands of the sling, but it doesn't seem to affect the constriction. Also, I didn't have any significant problems loosening them up afterwards. Mostly just running a gloved hand over the outside of the bury did the trick.
YMMV, but I think they have been tested sufficiently to be considered reliable in winter.
“I think that when the lies are all told and forgot the truth will be there yet. It dont move about from place to place and it dont change from time to time. You cant corrupt it any more than you can salt salt.” - Cormac McCarthy