I really wish people on this forum would stop saying SIONS abilities are telegraphed and easy to dodge blah blah.
That isn't the issue.
At lvl 2 a Sion Q and E combo can chunk you for 30% at least.
With resolve tree and W he gets very tanky early. Far quicker than anyone else bar chogath.
He can harass with E.
His W shield mitigates damage allowing him time to CC you and disengage without taking permanent health loss.
His Q alone is arranged minion wave clearer.
He can push the wave under your tower and use W to take tower aggro while he Qs and Es you.
All that said I don't think Sion is broken overall. He's just so strong because of the rune system and damage creep. Even as Garen he destroys me. My E is useless early. What can I do? To get to him as a melee I need move around minions and even Q is telegraphed and useless. He just Es me and Qs me from safety behind minions, has insane damage early and is unkillable late.
You don't really beat Sion you just hope your jungler has the brains to try and help you survive the early.