Kishimoto doesn't get to decide what of his writing is good or not? That's the audiences job.
His plan did not succeed and he was an absolute pawn of Kaguya/BZ. He was a manipulated middle man for her revival, his plan would've turned everyone into white zetsus.
'Evidence'. His motivation is a direct rip from Obito's from an audience perspective.
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His 'peace' motivation contradicts his love for battle and bloodshed. He turns evil in a completely unconvincing way that makes him look like a stereotypical pantomime villain.
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Not to mention Madara simply does nothing interesting with his short time as main villain. Boringly defeating characters right, left and centre seems like pointless padding and filler in the already overly long battlefield.
Kishi saying he doesn't know how to defeat Madara is literally him saying he has written a poor villain and written himself into a hole.
He's a bland antagonist, which is a shame bc he was originally one of my favourites. If you think strength is what determines a good character then that says it all.