The title link goes to the personal account of a CCA employee who details a lot of the problems that seem to be inherent to their hiring and staffing processes. CCA fails to conduct adequate background checks on their employees (which often results in them hiring ex-felons), they don't test incoming guards for their capacity to handle such a high-stress job, and they don't train their guards nearly as much as governments do. This resulted in poorly-prepared COs that smuggled in contraband, compromising the security of the facility, had inappropriate sexual relations with prisoners, and bragged about having spent time in prison for aiding human smugglers. As the author himself says, "I worked with officers who had no business being officers because of physical limitations or an extreme lack of intelligence. Such facilities [private prisons] are not safe, neither for officers or staff."
But this is no isolated incident; I have spoken with guards who worked for all different private prison companies, in different states, and all the stories sound very similar to this one. Check out the comments section on this article for a private prison guard named Pedro who discusses why he thinks privatization of prisons is "horrible for the overall well-being of the criminal justice system."