OneDumbG0
Find Your Own Fire
^ I've been thinking that as long as the attack doesn't involve stopping or halting Juggernaut's momentum, then his forcefield doesn't turn on. So if you try to slash him, you strike the armor directly and his armor is relatively invincible (but capable of being damaged). But if you try to push him back with an attack, i.e., Mjolnir toss, then the forcefield's up and he's virtually invincible.
So his forcefield (which makes him virtually invulnerable) is directly tied to his momentum enchantment.
I think this explanation helps explain certain circumstances, e.g., his armor getting shredded by claws contrasted with his forcefield completely negating a Mjolnir-toss. The former has nothing to do with momentum enchantment and the latter does. So he only seems more invincible to the latter type of situations than he does to the former type. But there're probably holes to poke in this No-Prize attempt as well.