It's very true. However, there are some frames of reference to judge by.
For example, Dilaf gives us the information that every one of his monks has the strength of a hundred men. That is the equivalent of fifty-ish Koloss, plus enhanced speed and other miscellaneous abilities. That would make for a very dangerous melee fighter, and the only way I see a Mistborn killing one is with a lot of duralumin and pewter, or some atium. A Feruchemist could probably kill one monk with ease, though, as he tapped all his metalminds to 120 times normal abilities. He would only have a few brief moments, but during that time he could crush a monk with ease.
So if the Feruchemist is going to take down the monk, that only leaves the Allomancer to take on the Elantrian. Without Atium or temporal external metals to work with, any hope for the Mistborn lies in either their greater mobility or ending the fight quickly. Mobility is a key factor, because the Elantrian would remain pinned close to Elantris, whereas the Mistborn could rove freely, perhaps drawing their opponent to a location where Aons have weaker effects. If that was not an option, probably the best bet for the Allomancer would be to burn duralumin, steel, pewter, and zinc/brass all at once, moving towards the enemy at a rapid pace while hopefully disrupting any Aon creation for a few seconds with crushing emotional pressure. If a Mistborn got into close-quarters with an Elantrian, I would have no doubts they could win a one-on-one fight.
Now suppose either the Mistborn or the Feruchemist were to sacrifice themselves, allowing the other to Hemalurgically endow themselves? That would form a Lord Ruler-esque creation, allowing the surviving member to both Allomantically burn and Feruchemically fill one metal. But which metal to choose? A Feruchemist who can burn Steel? A Mistborn with Mental Speed augmentation? They would have to face both a Dakhor monk and an Elantrian alone, so it would have to count. Maybe a Gold-healing Mistborn?