I don't know who's been saying that susanoo is invincible, but its not it's only the ultimate defense. Only itachi's susanoo should be called invincible when combined with yata mirror. I would also like to point out that madara's susanoo wasn't fully completed because you could clearly still see it's ribs.
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Look at itachi's "Complete susanoo" do you see any rib cages there?
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Now here's Sasuke's "complete susanoo" do you see any rib cages on his?
So we can say that madara was not using his "complete susanoo" because we could see the the ribcage. But does that mean im saying it was incomplete? no of course not because an incomplete susanoo looks like this:
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There's no skin only bones.
But if we look at madara's we can see that their is a layer of skin in the upper body of susanoo:
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This means that madara's susanoo was forming into an complete susanoo but madara didn't want to process it further then being in a partial complete state.
If you don't believe me then look at itachi's susanoo:
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You can see the upper layer of skin forming on itachi's susanoo, just like madara's if you compare them.
I believe that madara isn't using his final or complete susanoo because he's only testing the kages power. He's lowering his defense but at the same time using offense to let the kages know that he's attacking back. Madara still isn't showing his full potential yet.