Why is Amumu Considered a "Trash Tier" Jungler?

I've recently just started maining Amumu and have climbed from Silver I to Gold II in the span of three days with a winrate of over 70%. It's super easy to secure objectives with him, he has really fast clears after his first set, he counters a lot of the meta picks such as Jax, Nasus, Lucian, and Ahri, and he's also really great at stealing objectives/setting up aces with his ult. I've been able to cheese 20 min barons with just my top laner since the enemy team never suspects it, and I always get the rift herald in every game I play. Amumu just seems like an all around good pick and not deserving of "Trash Tier"; if anything he should at least be B tier due to his early vulnerability. But that's easy to workaround too as long as you keep vision in your own jungle; I for one always ask my top laner to ward whatever buff I'm not starting and then if I see the enemy Jungler in my jungle, I just go to his side and steal his buff.
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