Literally everything stops sion's passive from doing anything. Sion can be stunned twice from the same CC like jinx's traps, ok, whatever. It's the fact that the most minuscule of slows can completely stop sion's passive from doing anything because anyone can simply outrun it. Meanwhile, Tibbers runs at you at 600 miles an hour/has more HP/Deals enough damage to secure her kills after dying ect. Nerf her tibber's reliability so that its equal to the reliability of a late game sion's passive.