People hate the strategy because you have to send a babysitter to the lane or he will AFK ult into the tower on cooldown. Him ulting to the tower means that if you're not already actively winning a fight near an objective, you are already losing the objective trade. So your team scrambles to either engage or to stop the AFK push. This is solo queue, so people are not going to be on the same page. and there might be a terrible engage. You might not be able to engage due to enemy team comp. Maybe half your team tries to go stop Sion, right as your tank tries to engage. Maybe your tank fudges the engage in the heat of the moment because they feel stressed to make an engage and then your team dies 5v4 and you hard lose multiple objectives.
and every time he ults, your team doesn't know that this is the one time he is ulting to 5v5 with his team. Regardless of his score, unless you manage to one-shot him before he hits your team, Sion's ult is an extremely effective engage, or at least a zoning ability, if your team dodges it completely.
and on the other side of it, Sion is quite literally inting. If the enemy laner is a snowballing champion, or if the enemy team has a burst mage with Mejai's, Sion will lose the game for you solo. Syndra, for example, with an early stacked Mejai's, can Q -> WQE your squishies for a one-shot.
It's not unbeatable. I wouldn't even say it's successful more often than not, even in solo queue. But it feels awful to play against or even to have it on your own team. It was never intended that a champion ignore [ A ) Enemy champions, B ) Enemy tower damage C ) His own minion wave position relative to enemy towers D) HIS TEAM ] as a strategy. This strategy should probably go away for game health, but I don't think they need to do anything directly to remove it. I think the upcoming indirect changes Riot has planned for the game will remove it.