The anime confirmed what I suspected. Madara's ribcage only withstood getting hit by massive ransengan. It then became a real susanoo and knocked the ransengan away before it exploded. The ribcage didn't tank it, it tanked impact of a ransengan hitting it, then overpowered Naruto with incomplete susanoo. The full scale of devastation wasn't delivered. If it did, the massive ransengan would look like this.
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A SM massive rasnengan from Jiraya was said to be strong enough to easily carve away an entire mountain if it exploded according to the databook. Naruto's SM massive ransengan might be comparable to TBB. There's no way that even a complete susanoo (the one after skeleton), can tank something more powerful than something that destroys mountains.