your chosen schedule is not always given and you might not get all the hours you asked for.
you don't get paid for overtime, you can't submit on website more than the contract's maximum hours per week (37.5hours for full-time and 24 hours for part-time), even when they ask you to stay longer, and this goes for both full and part time.
if there's not much work or it's a slow week you won't be asked to come to work or be given schedules to select.
bad management, if you choose to select a schedule to work 8 hours from 8am to 5pm, you could be given less than what you asked for, for example 9am to 11am and 4pm to 5pm with nothing in between.
some of the employees performing your same task will just be doing the bare minimum, or just walking around doing nothing and getting the same pay as you are or more.
minimum wage.