There is plenty of instances where enemies will get stuck on doors, passages, etc. They can’t get to you, and you can cheese them from afar. Same with Crucible. Especially with the bows and range.
Enemies should not stand idle while you are damaging them, it is basically cheesing/exploiting and it’s not fun. They should move, try to get to you, and all enemies should be able to travers floors and ladders, or move out of the way.