I don't see why the large wood golem, with intelligence to boot, would be unsuitable for combat. It should be quite deadly.
Other than that, like I mentioned already, warfare is largely about logistics. I don't blame the author too much, though, since Japan is a deeply pacifist country, where the majority of people have zero connection to anything military related. So, an average Japanese person, like the author of this work, wouldn't necessarily have any idea that logistics win a war. Japan's fate in the WW2 was also hugely affected by the USA wrecking the Japanese logistics. In fact the outcomes of most campaigns in the WW2 were dictated by the logistics succeeding or failing. That's just how it goes. Elt's magic is very good for logistics.