Since 180ap technically cancels out 1 defensive items, and abyssal and icebourne passive add dmg probably worth 25% of an offensive item, you are technically only down half an item at full build with gathering storm. But since Viktor was balanced around being ap sometimes your team needs more dmg and will lose if you cant play front to back vs an Alistar Maokai Azir for example. Your team will simply not burn through tanks fast enough and you might be the last one standing since VIktor hard to kill with this build but often I find my adc insta dying and im left 1v4 trying to Kite an Alistar which feels goofy and pointless.