“I think a better way to regard the Japanese Army is that, of all the participants in World War II, they were the only one that had not learned the lessons of World War I.”
The early success against the Americans in the Philippines and the British in Singapore indicate that they had evolved since WWI, but they just “plateaued” in 1941. Few new weapons or tactics, just more of the same; they’d been fighting in China for years at that point, and decided to stick with what worked in 1937.
Looking at their equipment (wrapped puttees, LONG rifles, local dependence on supplies instead of prepackaged rations) and tactics (banzai charges against machine guns, minimal artillery), I still think they hadn’t learned a lot from WW1. But then, as Nomonhon demonstrated in 1940, they were very good at conveniently ignoring unpleasant truths.
I think their primary advantage in 1941 was that they had been fighting for five years already, they were up against comparatively modest and quiescent colonial armies that couldn’t be resupplied or reinforced, and they had the advantages of surprise and years of planning. Of course, even that didn’t always help them, as General Homma learned the hard way with the Bataan campaign.
Their biggest failure, of course, was that they had planned on a quick conquest of the resource-rich Indochina and Malaya/Dutch East Indies region, which forced them to take a lot more territory (i.e. Burma, the Philippines and Central Pacific islands) as buffer zone. Not only did they manage to overextend themselves, but they got drawn into a long, logistically-impossible-to-support war, which was the last thing they were prepared to deal with.