The missing advantage was cruiser survivability

A cruiser is the lowest general-purpose surface combatant expected to absorb damage and remain useful in battle. The Giussanos had six-inch guns and high speed but lacked the armor, subdivision, and torpedo defense that normally distinguish a cruiser from a destroyer. Their long, narrow hull is contrasted with the more balanced Arethusa, while the related British design study also produced the Tribal and Dido classes. Dido's dedicated anti-aircraft role is criticized, though it still appears more coherent than a Giussano whose cruiser label lacked cruiser survivability. Category, mission, and protective structure must agree if a ship is to remain useful after being hit.