The work is far too difficult for the measly, barely-above-minimum wage that they give. You must work Monday to Friday but are only guaranteed 15 hours a week, however you are expected to have full time availabilities (you could be working anywhere between noon and 7pm). They demand nearly impossible goals that are basically never met by anyone, such as 950 packages sorted per hour in small sort (when most average 600-700) or 250 packages when loading trucks (which I don't believe I've ever seen reached - closest was just over 200). Goals like this that require so much effort are insulting when they refuse to pay anything above a poverty wage to the package handlers. Instead of a decent wage, they'll often buy pizza or enter you in a raffle to win an iPad or things like this, but these things do not pay the bills.
The plus side is that there is the chance for mobility, but few people I work with are even interested in positions such as supervisor as they all see how much stress they go through. Everyone is stretched thin since the company only wants to have the absolute minimum number of employees available at any given time.