Fog is the worst for me, especially after you've committed the time (and pain) to get up into the mountains and then can't even glass.
Fog can be frustrating no doubt, but I have also used it to get across some open areas that I otherwise would have been belly crawling across, or taking a 2-3 mile, several thousand vertical feet detour. Just this past August in the Talkeetnas the fog nearly made me miss spotting my target ram. I barely saw a fuzzy sheep outline in the distance and the fog then immediately worsened before I could get a good look at it. I chanced a 2 mile hike to get closer and see if it was him but used the fog to cross some open slopes. This obviously required me to know the area, but it worked and I got the ram.
