is actually ban worthy in OFA.
He doesn't have that much kill potential early but it's really hard to get kills on them (5 Sions) early as well because of their zoning and CC potential with Q and E.
Once they have a couple of ranks on W you will not be dealing damage if you're a squishy carry, you simply won't kill enough of them. If you are a squishy carry type then they will cut right through you with their decent base damage with Q and W and later their ult.
You will be forced to Flash out of two Sion's charging their Qs after they slow you with their Es just so that you don't get CCed and die from their high W dmg throughout the whole game.
After they get a Black Cleaver they can all literally ult towards your towers and autoatatck them, because they are so tanky with items like Randuin's, Banshee's Veil and what not they will take ages to kill by 20 minutes. Imagine trying to kill 5 super tanky Sions at 20 min mark when your tower can barely kill one.
If you do manage to kill them their passives will happen and they will take down the tower, they can keep doing that every time and you can't stop them.
He can do this against any champion in the game.
Seriously, try Sion with a couple of mates, it's OP as fuck!