Sion's W is very small, so it's compensated by his Yasuo-like minion dominance E, which provides a healthy slow/knock up (knock up has a long duration) along with his Q, which does a pretty hefty amount of damage if used right.
Not to mention, that Sion W is a shield, and if he IS a tank then it's not gonna go down easily because it scales with his HP. Same kind of "punishment" from this annoying tree
who has a Root and a tiny knockup and an small AOE slow (i mean AOE like it deals damage to multiple targets) that does a fair amount of damage, even when tank.
has dumb "punishment" abilities too, like an AS slow with another slow and FREE damage on auto's when you catch up to them just for landing a pretty big AOE slow and/or a point and click slow, and don't get me started on the super fast AOE knock up "nuke" (depends on what you build)
has a ranged hook(stun) engage with a small knock up positioned near him in a fat line and one of the STRONGEST SLOW'S IN THE GAME. Not to mention how he has the ability to put his ADC next to him the size of the middle part of the bot lane.
Ok, what about Poppy?
has a pretty huge range stun (not to mention her stun is a tiny bit longer than it looks. Sources: mastery 5 poppy here) that need's to be positioned though, and it's not "easy" either. Has Q that does maximum HP damage that can enable you to be basically Susan
and deal pretty hefty damage without building damage. Has an anti-dash W with a pretty wide range, especially in the jungle. and a knock up (not as useful as a normal knock up) when used immediately. Not to mention a ranged attack that is a buffed Corki auto attack that provides a shield depending on your health.
And where does that relate to? Sion's W.
Tank's don't have punishments, they have things to get to you that somehow make you deserve "to be punished."
Just my 2 cents bro