We can start with the obvious I suppose. Whiteout and loss of tail rotor effectiveness.
On an overcast day in snow covered hills there is little or no depth perception when working near the ground and this has been the primary cause in far too many helicopter mishaps.
Loss of tail rotor effectiveness is something most mountain pilots will have on their minds while conducting high alt ops, particularly when operating near gross weight for that altitude. I can personally atest to being very short final for a peak or pinnacle on a few occasions only to find my left foot can no longer go any further forward. Round about that time the helicopter simply wants to start rotating to the right. If one is lucky and the aircraft gives enough advance warning, the pilot may have room to lower the collective, fly away and try again.
I can't comment on high altitude resuce ops though so will leave it at that.
My two cents.