They all deal % missing HP, and only a few are assassins.
God forbid Kha'Zix gets his % missing health damage back.
Not even beginning to mention the amount of champions (namely assassins) that deal % MAX health damage.
Zed, Vi, Jarvan, to name a few.
DJ Di03 votesNidalee doesn't deal %hp.
Only Play Darius2 votesIt allows damage that is relevant to tanks without nuking squishies.
%hp is the way forward in terms of balance.
DrProfVibeCheck1 votesFear the CarryBear 
ProfDrDeath1 votesNidalee, Volibear, Riven don't deal missing HP. The damage of one of their spells scales up to double/triple with the enemy's missing HP. That's a difference.
J4 is current HP.
And for the rest of them, most of them are either not superspammable/DoTs (Fizz, Garen) or the champion has been repeatedly run through with the nerfbat for it (Elise).
