I'm not downgrading anything. I'm just interpretating Madara's saying. Maybe it's because you're a Madara fan that you simply can't see it with eyes?
To rival something definition:
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Susanoo attempt to equalize the power of a Biju if not surpass it. It's just an attempt to rival a Biju power, which means it's atm equal if not weaker
He said it can smash all things in the universe, then added it rivals Biju, which means a Biju(Shukaku) could do the same thing. The hype is also including Bijus considering the context
Having a comparable power to something doesn't mean they would be equal in a fight. I never said Madara's PS would lose to Shukaku. It's their power which is comparable if they are put one against the other, but with Madara using it, he would most likely overpower Shukaku despite his weaker if not equal fire power. Humans have defeated Bijuus in the past without having the same fire power