I get what you are saying and if it wasn't for it being black zetsu i would agree.
What i believe actually happened was more or less a transformation rather than over powering. Madara simply was activated to complete the next stage which was Kaguya.
Black Zetsu was essentially the trigger for Kaguya to be revived.
Much like when the tailed beast caused the Gedo to violently transform into the Juubi after Obito triggered it.
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If you consider the Gedo transformed due to all the pieces of the puzzle being completed.
You could also argue that Madara combined all the pieces for Kaguya to be revived. Black zetsu simply activated it.
While I troll about Black zetsu "Over powered and suppressed" Madara I honestly believe it was a mutation that Madara simply could not stop. Just like the Gedo couldn't stop its transformation.
Cocoon to butterfly concept.