Depends very much on the assembly line. Some are quite reasonable, especially in smaller firms where you don't have a completely detached management in their ivory tower passing efficiency edicts for grinding through disposable anonymous minions. You can't abuse your staff too much if you have to sit next to them in the canteen! It's not exactly stimulating work and obviously not a rockstar salary, but it's comfortable enough indoor work and you don't have to pee in bottles and have a psychopathic delivery schedule every day.
With gig work you get none of that.