Wuju style's passive and active effect.
Master Yi's bonus attack is literally a second strike. All on-hit effects apply, and it can crit. It is literally a 1:1 ratio on his AD. By the end of the game it is not hard to be running around with Master Yi having ~400 AD just between his gear and Wuju Style.
Diana has no AD steroid, and it certainly cant be activated to double over what the passive does. Diana's 3rd hit cleaves for a flat amount of AP that gains a bonus equal to 60% of her AP.
They're apples and oranges. Diana probably needs a nerf, and I'd like to think that her passive would be a good place to start. This in no way means Master Yi's passive is somehow bad. It's apples and oranges.