Key aspects of 'The Mule' based on a true story are multiple. First, the character study of the man who becomes a mule is important. His motives, his naivete in some ways about the drug world, and his later realization of the seriousness of his actions. Second, the movie depicts the power and reach of the drug cartel. They are shown as calculating and yet also able to take advantage of the weaknesses of people. Third, the movie's setting and the way it shows the different environments the man travels through while transporting drugs add to the overall atmosphere and understanding of the story.
The main aspects include the journey of the protagonist. He starts as a normal, if a bit of a misfit, old guy. But then he gets involved with the cartel. The movie also shows the consequences of his actions, not just legal but also on his personal relationships. And the way the real - life story was adapted to show the moral ambiguity of his situation is also a key aspect.