It's hard to summarise working for Costco as you will have a different experience to others depending on which department you work for.
For most, the departments are difficult to work in because of similar reoccurring problems such as short staff and reduced hours which leads to low employee morale.
There are many managers in each warehouse and whilst not all of them are 'bad', there certainly are a lot that lack in basic people skills and some are often promoted into management just because of out-of-work relationships etc.
If you choose to work there whilst you study, or just for 1-2 years then your experience will be okay, depending on the dept. However if you join wanting to progress and make a career, you should note that most are promoted through 2 ways: Working in every department (Especially the shop floor) and/or becoming friends with one or more of the managers.
When joining the best advice would be to keep hold of your Employee handbook and study it carefully, and make sure it applies to employment law.
Management can use the book against you for whatever reason and having knowledge of the handbook and basic employment law can help defend yourself.
Hardest part of the job is simply this:
People can be very difficult and incredibly stupid.
Hourly pay rate, staff members
Bleak working conditions, management behaviour and conduct, difficult customers