Can only speak for how things are for me, obviously; and, I've only worked preload so I have an implicit bias. Also, I work off a belt, not cages - I have seen the cages from a distance, never worked them though.
Preload is ... both great and horrible. There are days where the operation, as a whole, adds up and everything works well - though, this is the exception rather than the rule. Additionally, some folks who come in the door have been awake for maybe ... twenty minutes, so they are really dragging butt and moving slow; this inevitably backs everybody else up and tends to piss the supervisors off.
There is also a lot of pressure put on you to perform - not just from the supervisors, but from the drivers; a lot of time the drivers will hold you responsible, rightly or wrongly, for the quality of the load. Obviously, there is some responsibility to be placed on how the routes are cut and who does it, but in my experience you will catch the initial storm from the driver when they see what they have to deal with and they are on their way to yell at a manager.
Some of this is applicable to all jobs at UPS I'm sure, but that is just what is off the top of my head.
edit: Then again, I really enjoy when I am able to work a minor miracle and perform a difficult work assignment to a high level of quality and with time to spare; that is just me though.