i used to work for amazon in a warehouse, i wouldnt say i was underpaid and the insurance isnt bad, i work in manufacturing now and get paid significantly more but my insurance is almost useless. But theres a gap between amazon warehouse workers, software engineers, and customer service, along with the other varying fields. I think there is an overall dissatisfaction of employee treatment though, there has been countless articles written about it and i dont doubt what any of them say.
as far as walmart goes, im not real sure, but from what i hear they are trying to turn themselves around from that minimum wage, crap starter job image that they have. I know people in my town are getting paid decent wages and good benefits. I did read an article a while back about walmart getting huge tatx deductions simply for employing people at minimum wage, making them more profitable overall, at the expense of the employees.