Repetitive tasks are the least of it, usually.
The most frequent complaints I hear about are the loud noises and the working in shifts. At my factory they have to work in 4 shifts: the shift is 8 hours, one week it's the morning shift, the fallowing week there's the afternoon shift and then there's the next week with a night shift. The 4th shift is 12 hours on Saturday and Sunday, 2 weekends, with the Monday to Friday being considered free days, but many of them use the "free week" as a time for getting extra hours and money.
That sounds about right. In some companies, there is mandatory overtime. 6 days a week for 9+ hours a day.