They are inflexible. You don't get to choose what shift you are on or the hours. If you have to leave 15 mins early to go to a doctors appointment or something it's coming out of your pay. You sometimes have to fudge timecards, saying a battery takes 7 hrs to change when the real hold up is other trades or logistics.
Usually a 4 day week from my experience, slightly longer days but well worth it for weekend lifestyle.