I strongly disagree. How does a Fullborn get through Shardplate? Shards aren't metal, they're spren, so I doubt a mistborn could push/pull them. I think the outcome could depend on how much investiture they have, true, considering that a mistborn with unlimited atium wouldn't really be in danger. And it matters about the mistborn's coin count, because otherwise they would be quickly grounded. And it would definitely matter the order the Radiant is. I think a fullborn would beat an Edgedancer, but lose to a Windrunner. A Windrunner could make quick work of the metal sources the mistborn could use to "fly", and then lash them to the ground enough times to render them immobilized. Then atium wouldn't be an issue. I think perhaps emotional Allomancy could be one of the only ways the mistborn could win. The battlefield would also play a large role in it. For example, fighting in a cave would be difficult for a Windrunner, but in that case maybe a Stoneward would be at the advantage.
Yeah, but I think that atium is the biggest thing in the mistborn's favor, if not the only one.