Confused on Sion's role

Maybe I'm just a total nubcake but, I'm kind of confused as to what Sion's new role is. His abilities aren't really conducive to tanking and is barely bruiser with his knock-up that people can easily walk out of before it is done charging. He still has his shield and his passive scaling with minion kills, okay fine. Then he has his E which has armor pen and a slow. Which is great I guess if you are support someone since if you go glass cannon ad you fall over. Yet his recommended build is like almost all tank items but, his abilities don't support that notion in a cohesive manner as other bruisers or tanks and certainly not as efficiently. His ult seems to be his biggest damage potential but that's if you can actually land it. How easy is it for an ez to E and Arcane shift to the side, or anyone to flash to the side and have Sion run past like a bull completely incapacitated by the mobility of enemy champs. It is great that his passive scaled with health or whatever but what does his passive matter if he doesn't do any damage?

tldr; Sion is clunky. His rework doesn't make sense to me presently.