The correct answer to this open question is the following.
The details that most convincingly highlight the problems in the meatpacking industry in the early 1900s were the following.
First of all, one of the most important issues of the meatpacking industry in the early 1900s was the poorly way to kill animals. A very savage and unhealthy way. The animals died in agony and that affected the quality of the meat. Another issue was that the health inspections were not as serious as they were supposed to be for sanitary control. Workers did not follow sanitary procedures. Another problem was that the workplace was infested with rats all over the packages of meat pilled up in very humid places.