Over the last several years, I've shot USPSA Level 1, 2 and 3 matches that involved shooting 5 stages to 12-14 stages (0745 hammer down and finishing around 1900) in conditions that included blowing dirt (low visibility and sand everywhere), 30-40 mph winds, 110 deg temps, 15 deg temps, rain, sleet, snow, and everything in-between. Never once did any of the crap produced from those environments keep me from using the dot to hit my targets where I wanted to. This was using Leupold DPP, Sig Romeo 3 Max, and Holosun red dots.
If you want a gun that you count on to run, you keep it somewhat clean, but most importantly oiled. You're going to want to keep your sight clean too. But, even when you can't and you're experiencing horrible conditions, the above isn't going to keep the red dot from functioning if it is a quality dot.
As an RO at those same Level 1-3 matches, I've also seen a lot of shooters in those conditions and their gear still running and their red dot sights still working in those conditions.
