Fleet Footwork AD scaling is ridiculous.
Fleet Footwork, 5 - 50 (+ 0.10 bonus AD, + 0.20 AP)
Early to mid game this Keystone is fine for AD champion. But as for the late game, even with 300 bonus AD it'll only heal you for 80 health.
Also, I always thought AP ratio were lower than bonus AD ratio, an AP champion can easily reach 150 health with this keystone in the late game, nearly the double of AD champion.