Plot hole: There is no reason at all why, being targeted by a few arrows by unseen enemies - a fire suppressed already by the salvo of their own archers - the Rourans would turn around their heavy siege equipment, away from the bulk of the enemy forces, and fire it, hurling a single heavy stone to the middle of nowhere when they have the whole rest of the army who could storm the rock the supposed enemy commandos hide behind, or the archers who could keep shooting - again, they proved to be completely successful. It also makes no sense that the all-powerful witch who made the warriors flee managed to do any of this, 'sneaking' by horse in the middle of the steppe.
Plot hole: The 'avalanche scene' in this remake is mind-boggling. For starters, the Witch single-handedly holds the whole Chinese army at bay splitting in a zillion of flying creatures - a power level completely inconsistent with the rest of the movie. To the annoyance of being pounced by little birds, the army gets in turtle formation, apparently just waiting it out. The Rourans somehow are ready for this and have a trebuchet set up - despite the fact that they are nomads, conquered the forts infiltrating them, and they were skirmishing a moment before. They throw flaming boulders with such precision that they are able to target each single 'testudo', multiple times, with the soldiers just sitting there with no reaction.To save them, Mulan is able to sneak behind them UNSEEN, by horse, with a bunch of extra helmets she somehow carried, set them in place, and fool them. Any of these convoluted operations would have taken an impossibly long time.
Plot hole: At the beginning of the movie it is said explicitly by the Chancellor that the Rourans have attacked 6 garrisons at once, disrupting trade on the Silk Road, which would, in his words, threaten the survival of the whole Empire. It's a bit odd considering that their assault relies on the Witch's abilities, and she can't be everywhere at the same time, but forgetting that; the Emperor to counter this urgent menace (Bori Khan slaughters everyone in the cities) decides to summon to arms literally the whole kingdom amassing a huge army. This obviously is a project that takes months (we even see Mulan taking days just to get to the training spot, and then they train long enough to become proficient in archery when they started off not able to even throw an arrow) and does absolutely nothing to stop the brutal raiding and killing, but somehow Bori Khan's plan is kindly waiting on Mulan and her buddies to train, despite being a plan based on speed, surprise and distraction.