Why nerf Roar of the Slayer instead of Soul Furnace?

Because Soul Furnace is pretty much fine °_ The problem was the endless harass that Sion could get out in lane with his E.

Because his E barely costs mana, has a long range through minions and has pretty high base damage. For his shield he has to be in melee range to do damage, has meaningful mana cost and can be destroyed before it detonates.

Because it was that shout that gave him too much trading power early on it lane for what it did and how little you could do against it.

Yes, lets nerf the tank for what a tank should do. 'slow claps'