SKS guards break when the temperature gets low in my experience; A ride in the -1 to +2 kind of temps plus a wee knock was enough to kill mine.
Not the experience I've had with 7 years off commuting all year round and only needed two sets of guards in that time. The front one broke at the fork mounting point after a couple of years. Coldest ride last year was -10 but regularly ride 20 mile each way commute in sub zero temps through winter.
I've found trimming the top dome off the plastic bit the stay slides through makes fitting a whole lot quicker and save a lot of trial and error. I then file the stay down so it's flat and smooth. Makes it much easier to get the guard very close to the tyre.