Icing is a huge concern. On a drone it just adds weight but it can add up.
Don't fly through clouds.
Flying near waterfalls is where you could find freezing mist. That can accumulate very fast.
probably not a big detail, but sometimes, obstacle avoidance works fine when leaves are on trees and fails when the leaves are gone from thin branches
not that I've ever encountered that problem because I never screw up....uh huh