I love hammer guns. But, my thumb did slip off a hammer once as I was cocking it when I was hunting in the rain, walking up on my dog on point. Of course, I had the gun at port arms with muzzle pointing skyward, but be aware of that going in. Since then however, I don't use hammer guns in the rain. I don't like to upland hunt in the rain period, but I do live in Washington...
There is also the safety issue of making sure to remember to let the hammer down after you cock it if no bird flushes.
Kirby Hoyt had a nice article in Shooting Sportsman a while back about quail hunting with hammer guns (he has quail running around all over his spread in Wenatchee). He cocks once on the rise and shoots, then drops the gun, cocks the second hammer and re-mounts. It only takes an extra second or two, but it is the safest method. Or,as someone on here once posted, "we're not at war with the birds."
