Then what were the Japanese good at during WWII? Just the Mitsubishi Zeroshiki?
And what were the Chinese good at?
The Japanese were not as advanced as the western world, they chose to focus on aircraft and the navy rather than tanks. They had a superb navy, though it couldn't come close to the rate of buildup the USN did during the war. Their aircraft was quite good at the beginning of the war but these were soon eclipsed by superior and more numerous planes. The Japanese army was the worest. They had dedicated people but their equipment from tanks to guns were universally c**p.
China imported or license produced all its weapons. There were no major weapons of indigenous design. Not even a lousy submachine gun. We had no real navy, the airforce was ineffective and the ground forces were disorganized and poorly trained. There were some very good small unit infantry on both the KMT and CCP side but these weren't enough to win big battles. What was needed was for the KMT and CCP to have a coherent grand strategy and use coordinated, aggressive counter-attacks instead of the containment actions in the south and harassment actions up north.
If the Soviet Union seriously supplied Mao with arms he could also breakout east across north China. Japanese occupation zones would be cut in two and supplies from southeast Asia to Japan would be interrupted. The Japanese weren't really strong on the ground. Just that China was extremely weak.