I have polycarbonate jugs/bottles that look just like milk jugs except that they're polycarbonate. I routinely have one inside a cooler along with other stuff that I prefer to keep cool. Don't know how they'd do if frozen. Everything has its limits.

One reason to use clean milk jugs for this is that they're cheap enough to toss after each use. Nothing says these jugs are intended for long term; with cheap jugs they could be replaced every week.