It makes him pretty much impossible to poke out of lane, and by endgame it's basically a Bloodthirster on steroids. I've seen an Aphelios facetank a Draven with bloodthirster without much issue. And honestly, no adc should be able to stand and trade with a draven or kog'maw, especially one with the utility and flexibility of aphelios.
He's actually squishy when he isn't healing for ungodly amounts with severum.
It could be as simple a change as making it so severum only heals and shields when hitting champions, so he can't just take a trade for half his health and be like Pyke and heal it all up seconds later.