Even if you built your deck to consistently reveal high CMC cards, you're limited to hitting only a single target at a time
Niv-Mizzet, the Firemind + Curiosity Sometimes, dealing infinite damage is just too easy to be called fair
There are other ways to get around protection, but they usually entail more niche cards like Everlasting Torment or Rain of Gore
I think it was a fine card for formats like Standard and Modern, because it didn't absolutely dominate either of those formats
Removing a shield counter when a permanent would be dealt damage or destroyed isn't the same as regenerating a creature
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