You Wonder Why Assassins Can Destroy Tanks
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Every armor pen item is a viable core item for AD casters.
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Every armor pen item has an affordable and effective build path.
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Pen/reduc calculation order makes it effective (albeit not optimal) to stack both flat and % pen.
This makes it incredibly difficult for anyone to itemize against them. The only strong itemization against assassins is Zhonya's, but it has little to do with the armor past laning phase. Assassins should be able to wreck carries without much of a problem, but not tanks at the same time. Personally, I think it's fine for assassins to be able to destroy tanks, but only if their build path sacrifices for it heavily.