The Dust-Brush of Master Primordial Unity of Yin and Yang's offensive capability isn't particularly strong; at best, it's evenly matched with Six Pure Bamboos and it's not exactly a killing-class treasure.
Moreover, the Dust-Brush is a magic weapon that Yu Linglong had confidently revealed earlier, so anyone who wants to face Yu Linglong will surely count the Dust-Brush into their plans.
That's why neither Ji Changsheng nor Yu Linglong pinned their final hopes onto the Dust-Brush before they moved against the Ten Thousand Demon Country.
You can't really treat your opponents as fools.
Of course, the facts prove that if Six Pure Bamboos hadn't appeared, Yu Linglong could actually have pushed down her opponents relying solely on the Dust-Brush, and she didn't even need to use it extensively...
That could only be attributed to Yu Linglong's overwhelming strength, as well as Ji Changsheng's version being far superior.
It completely stupefied their opponents.