User blog comment:Witchking99/Riuus, The Vermilion Royal/@comment-24420226-20160516150029

Seems countereffective to have current health scaling on an ability that actively limits an enemy's health. Also the health needed to activate his W and the health limit values become so disparate at higher ranks it just becomes a more powerful version of greivous wounds that effectively negates healing.

Also, like Chase below, I don't see a terribly big reason to use his ultimate on an enemy outside of laning phase. At mid and late game, putting this on your ADC will net you dumb levels of healing - think of the crits!