Sion is too unreliable

All he does is CC and tank damage, so why does his cc have to be so easily avoidable? If he misses his CC then what can he do? He doesn't burst, he doesn't dive, he doesn't duel, he doesn't split well, he doesn't shred towers.

His Q and Ult have huge windows of opportunity for counterplay, unlike those of many other heroes who also can do more than just CC and soak damage.