Talon has tools Diana doesn't, and vice versa.
Shes a ap talon that is not item reliant and can stay in fights and thats why she is poorly designed and broken.
She's similar, but not quite to an AP Talon. She has the shield primarily because her ultimate takes her into melee range, and she needs to survive. Unlike Talon, she can't go invisible (convenient that you forgot about that). Unlike Talon, her E doesn't increase or do damage, it synergizes with her W and passive by bringing opponents into melee range instead of blinking her like her ult and Talon's E does.
Apples and oranges.