PLS! Do your selfe a favor and kick those useless %Yellows onto the next dumpster!
Ever calculated their actual value?!?
Take ScalingHP Yellows instead - but keep %Quints. That's not just better in terms of power but they also scale with each other.
Besides that - yeah stacking HP is strong, however I'd only recommend it on Champions that actually use Health in one way or another - Cho, Naut - you know. Stacking Health on anyone who doesn't scale with is countered quite easily by all the %-shit that's flying around.
(Ever seen a Kindred with BotrK? x'D)