If you are management, then dealing with the union is the biggest downside. The union is allowed to have too much power, often leaving managements' hands tied most of the time. From as little as telling an employee to "hurry up," can cause an employee to file a grievance based on "harassment". Often employees will look for any supervisor moving a box or breaking a jam and will file a grievance because the suprvisor is "stealing" work from union employees. From what I understand this a little more exclusive to the Ontario Hub, and I understand it is not as bad in most other hubs.
Upper Management does not seem to care about their part time supervisors to the effect that often times they will not defend against the grievances filed simply because they may have a good relationship with a few of the shop stewarts or because they don't want to deal with arguing. A bad relationship with your lead supervisor or manager can make it worse.
Raises for Part-time supervisors are supposedly based on a combo of the shift manager or full time supervisor and a peer review. Most of the time 2 to 3% is the common raise given once a year. However, this is where either being buddy-buddy with the Manager or Full Time supervisor comes in handy so you can get a 5 or 6% raise. If you are female and somewhat attractive, it works to your advantage as you will often receive larger raises and more opportunities to advance faster without being necessarily qualified.
Part Time Package Handlers only receive 8.50 an hour to start and need to pay union dues and most importantly their initiation fee. The first 4 months of employment are very difficult because you get a small paycheck after all the dues and fees are taken out AFTER taxes. Checks less than 100 bucks a week are very, very common.