What is the most difficult stuff to move in your opinion. And Mud does not count.
Gumbo for answer number one,,,
Stupid bureaucrats
Wet Churt will wear you and a machine out.
Large stumps, rocks, bamboo, deep concrete foundation footings
Water and slop, from a pond bottom. Up and over the top the slope. That is no BS either. I did it many times.
Some difficult stuff is chewed up cars metal chips. Easy stuff rocks.
Trying to fine grade a material that has 3-6" rocks in it.