Let's be clear about something:
Zelda bair is the single strongest (non-special) aerial in the entire game, by any metric.
It also hits on frame 6.
This means its knockback-relative-to-startup is chart-topping; the only competition is extreme-case moves like Rising Uppercut, Rest, and Boost Kick, which have far less range and far more endlag.
Zelda fair, which is weaker and hits a whole 50% later, is still incomparably stronger than any other aerial you can consider; even the knee.
Yeah, the knee has a much better sourspot and late hitbox. It also doesn't hit until frame 14, has 5/7 frames more landing lag, and 6/8 frames less safety on block. It also can't be done out of SHAD.
Zelda bair (and its little brother fair) is an extreme proposition: You get the strongest aerial in the game, at an absurdly fast startup, decent range, and workable landing lag. The price you pay for this (again: the best) is a strict hitbox, dismal sourspots, and a poor autocancel.
What holds this monstrocity back is not the weaknesses of the move itself, but the context of it being held in check by a 6f jumpsquat, average airspeed, and a moveset that provides no reliable mixups leading into it.