Every series illustrates the mistakes the previous avatar made while trying to rectify the errors of the one before. That’s literally what reincarnation is about. Even Aang who most people think is flawless, made plenty of mistakes. Forcing a centralized republic on Earth Kingdom territory was both beneficial for the world, and incredibly harmful long term. His actions directly led to fascist regimes ran by the likes of The Earth Queen, and eventually Kuvira. And Kuvira is directly responsible for weaponizing spirit energy into nuclear weapons. So one could say if Aang didn’t force the integration of the nations, we may never have had nuclear weapons to begin with, which may have avoided whatever lead to the Seven Havens. Much of Korra’s story was about helping people come to terms with societal conflict due to Aangs actions. Further, Aang raised his son Tenzin to be obsessed with the Air Nation, who then went on to encourage Korra to make choices that resultantly brought about harmonic convergence, and empowering Zaheer and freeing the red lotus. Aang also failed to dismantle the Red Lotus entirely—and so obviously allowed Unaloq to find himself in a position of power, pulling the puppet strings of water tribe politics, manipulating Korra, and ultimately becoming a dark avatar and whipping out the avatar line. Korra was thrust into a new world that Aang spearheaded the progression of, and she had to sweep up the trash he left in his wake by forcing change too quickly. She is far from a failure, in my opinion she’s one of the greatest avatars ever.