Also, from that same article about Pentagon vacancies:

The former official added that Shanahan’s indecisive style, lack of experience in Washington, and failure to understand the role of Congress may drive some Pentagon officials out the door. Shanahan worked at Boeing for more than 30 years before taking the deputy secretary of defense job in 2017.

“Secretary Mattis was the smartest man in any room, but he sought dissenting opinions and listened more than he talked. By contrast, Shanahan believes he’s an expert on everything and routinely dismisses divergent or contrary views,” said the former official.

No wonder Trump loves this guy so much.