Fine, Hagoromo's statement can't be disregarded. However there was no proof of Kaguya's power until Naruto and Sasuke tested her. They couldn't have waited for Kaguya to reach her powers to full extent. Of course, Naruto and Sasuke failed because Kaguya is a brand character and the final villain, so there was no way the duo could succeed against her.
So Kakashi told her about the second rinnegan, but she didn't listen to him, did she? She failed to destroy the other rinnegan in Kamui dimension, that was the task she was assigned with. I won't bash her for failing to destroy the rinnegan.
But even if she didn't know about the second rinnegan, Sakura had already seen Madara in action for a long time. How could she think she had any chance against Madara? Sakura was extremely overconfident of her new powers. She thought she had finally caught up to Sasuke and Naruto with her Yin release. It shows her inability to comprehend and assess power level of other people and herself. She could have thought of a strategy on the sideline with Kakashi, but no she's too smart for that, right? To bring her back to reality, Sasuke had to remind her that she was useless under his susanoo.