so what do you think is most mentally or physically challenging job at ups then??
As the others touched on, and my opinion is that every job's different. Pkg driver, unload, sort, it all depends on what work you're doing in the moment. I've done pretty much everything outside having a route delivering, but every day is different.
One christmas back in 2004, management gave me (2) 24 footers and a budget rental to load. 3 cars that size was very very difficult, but it was only for 3 weeks and since the job was physically impossible, I just let the work build up until help came over.
Now does that mean preload is the "hardest"? No way. The hardest was a busy sort aisle belt I worked for about 5 months. Management had no idea how to distribute the packages remotely evenly, and we'd get like 35-40% volume on this one belt. I swear for about 4 months, myself and this co-worker missed about 20% of the split and then had to go around and sort out all the misses! That was hell on earth, way worse than any preload situation.
But, the most difficult job, mentally, is at the customer counter. When you have 3 people screaming at you because their package isn't available. Try doing this 8 hours a day and not telling someone to 'friend" off or the like, not once.

Even when they tell
YOU to. Having to talk down people that literally want to take a shot at you, or having a line of 30 people and you by your lonely self to handle everyone's problems.
It's apples to oranges. Every job's good sometimes, not so good others. Some are more phsyical, some are more mental. Make the best of what you have.