It's been mentioned in this thread, but do we know if Amy actually invented that ballista? The text bubble just says it's a new weapon. And I have doubts she knows enough general knowledge to design one. Is it mentioned in the LN?
Just looking at that monstrosity on page 13, I can't tell if it's just horrible or worse. It has 2 large side wheels, and 2 smaller perpendicular wheels. I can only assume that's how they're rotating it, turning the entire thing with pure human muscle power with those extending poles. It could probably work as a siege weapon, but that's probably not even hitting a moving horse at range. Feels more like they converted a normal ballista and made it "portable".
And even the armored conveyances in panel 3 are based on a bad conception of what they're fighting. It looks like a modification of armored battering ram structures. But with big wheels, to protect maybe a small squad of people. But also, entirely impractical other than as mobile arrow defenses. Angled metal front and a metal roof. Probably deflects most fire arrows, but doesn't look big enough to fit horses, so pure human power again. So tired porters moving that and leaving even less room for actual troops.