Considering how cheap the mirrors are they seem pretty sturdy. They are made from polypropylene which is a fairly strong plastic, same stuff they may batteries cases of. As far as the magnets go, I got the stronger of the two options I found. I really had to put my weight into it to position them, I doubt they're gonna rattle off.
It's been a pretty light winter here and not a lot of snow in the forecast right now, no significant snow in the next 14 days and unseasonably warm. Might be a while before I run the tractor again, but I will let you know if I have any issue when I do.