His worst failure by far, was the Varrian disaster of 9 CE, which cost Rome 3 legions. Augustus had wanted to move the Roman frontier from the Rhine to the more easily defensible Elbe but Varus was beaten trying. Augustus decided to live with the Rhine frontier. One could argue that another failing was his unwillingness to build a bigger army, try again, avenge the defeat and attain the original objective. But Augustus was mainly concerned with establishing peace and stability; evidently after all the great conflicts of the first century BCE he didn't have the stomach for a major new war. Some have criticized Augusts for cracking down on people like Ovid. This they say, stifled creativity in Rome.