For what it is worth here are some observations I have made and some that have been related to me by other match shooters.
I'm not sure how much opportunity the "average" reloader has to really experiment with loading in the winter and to compare it whith what he loaded in the summer. Before this post I went back over load records I keep dating back to 2001. Anything before that is not on the computer.
Around here we get to shoot in the kind of weather extremes that Hodgon likes to talk about when they advertise their "Extreme" powders. I've used two of what they are now calling extreme powders, Varget and H4350, and found them to be no more, and in one case less insensitive to temperature and humidity changes than other powders. When you load and shoot 300+ rounds per week for practice and match you can't help but get a some idea of what works and how. I don't load that much on a weekly basis anymore, but I get load a lot, though not as regularly as that and I talk to the guys who regularly beat me at matches to try to get any advantage I can.
I like and use H4350, but found it to be no better or worse environmentwise than some other powders I've used, including RL-15 and W-760. There are not a lot of other shooters using 4350 in match loads here, so my data and impressions of that is stricly my own. There was a spike of interest here in Varget for small bore, but one winter pretty well took care of that. I have seen posts by people who think Varget is the greatest and I am sure it works for them. Most of the people I know here went back to H335 or Vihtavuori N-133 or 135 and still have 2/3 full cans of varget around somewhere, as did I until I gave it away.
While not all the complaints about the Varget had to do with temperature and humidity considerations it was one element many of us found to be a problem, especially those who load at the match. This is not to say that adjustments do not need to be made in loads to be used in cold, wet weather with the other powders mentioned. However, there is only so much time you can profitably fool around with a powder before you dicard it, and some powders just seem to be inconsistent in density from the start. For some of us, in some cases, it may be a matter of preferring the devil you know.