I have smoked in -20F with zero issues other than my "Fat Volcano".

It was so cold that the fat was freezing pretty much as it was dripping out of the smoker which is what caused the "Fat Volcano".
From what I remember my et-733 worked just fine too.
I smoked some Ring Bologna last weekend in roughly -8 or so weather and there must have been a fair amount of humidity in the error because my et-733 got really frosty, but worked just fine.
As far as my #3 getting to and holding temperatures, I would say it probably took a little longer to get up to temp. But, after it got there it had zero issues staying there.
One other thing to note, is that unless you prewarm your probes (smoker and et-733) with hot water, blow dryer, etc., they won't start registering until they hit about 30° or so. But, if you just let the smoker heat up it will eventually start reading once it hits that 30° point.
BTW, my -20° butt turned out awesome just as they always do.
Good luck on your smoke!