If you want to compare
to
purely on grounds of reliability, then sure, Malphite is absolutely more reliable than Sion. However, Sion has far more crowd control and similar levels of damage and tankiness, and while his ult is more difficult to use, it can be used to initiate from practically the other end of the map, and has a much lower cooldown.
Overall, I think Sion might be a bit on the weak side right now, mainly because he was nerfed at a time when tank items were too strong, and was left in a slightly unsatisfying state thereafter. I'd personally revert the nerf to his shield detonation time and bring it back to 2 seconds, as it's what kicked him out of the jungle (alternatively, he could lose the damage reduction on his Q against monsters), but other than that he's not in bad shape.