Sett W needs to be nerfed
It goes beyond this ability slapping like a brick. The way it synergizes with his kit destroys counterplay. The counterplay to juggernauts has always been along the lines of avoid them or poke them down until you can kill them, but Sett's W just throws that out the window. For starters, his W is relatively long-range and practically impossible to miss. What this means is that he probably pokes harder AND more reliably than you. His poke being so long range then makes avoiding him(or his damage in this case) incredibly difficult.
THEN add his passive(increases HP per sec as his health lowers) into the equation and the end result is that you also cannot out poke him. Sett wants you to all-in, so that's obviously not an option either. Imo, Sett leaves you with NO optimal recourse. The only way this can be changed is if either the hitbox or dmg of his W were to be gutted.