What you said was a quote from Kakashi which is Partially true. Kakazu being this elite powerful akatsuki fighter losing to people who are meaningless to him, younger than him. But what can you say about Madara and the Kages?
Are you kidding me? LOL fail man. SO6P was able to defeat the 10 tails, seal that sucker up in him. After that he was able to split the 10 tails chakra into 9 bijuu's then use Chibaku Tensei which is now the moon, on the 10 tailed bijuu. His Chibaku Tensei is enough to show you his power. You think Nagato's usage of the Rinnegan was good, look at SO6P, His Rinnegan stated by NAGATO was FAR BEYOND HIS. The Bijuu's looked up to him. Nowadays in the bijuu's don't have anyone to look to besides Naruto now, that shows how much respect theBijuu's itself had for him.
SO6P was the first to understand "chakra", first to perform ninjutsu. He could use all Elements with Rinnegan. He had Yin and Yang, used that to create the 9 bijuu's. Created Form from nothingness-- sounds familiar to Izanagi, and possibly Izanami if we ever see it.
The uchiha's and senju's combined = SO6P. SO6P had two kids, one that became the Uchiha and the other Senju. The Uchiha's possessed the Eye's of the SO6P. Senju had the Body of SO6P. As you can see when you add the formula up They're clearly OP-- Madara.
His weapon makes him invincible if he isn't already. Bashōsen, Benihisago, Kōkinjō, Shichiseiken, and the Kohaku no Jōhei. Those we're originally his. Strongest Weapons, and he weild it.
Summary upon the powers of the weapons if you dont recognize it:
By combining the Benihisago, Kōkinjō, and Shichiseiken, the Sage (or any user) can capture the word soul of his victims, and then draw them into the gourd and seal them away for good. The Bashōsen can generate any of the five basic elements. The Kohaku no Jōhei can seal anyone without the elaborate procedures used for the Benihisago.
More info upon the SO6P on Narutowiki which i manage to pull some off, or you can back track the manga