Granted, we are talking about the KMT at the beginning of WW2 and it was the CCP that we are talking about during the Korean war. I also understand that the CCP had better morale and Soviet arms and quite a bit of actual war fighting experience during WW2 and the Chinese Civil War.
But if you look at the elite German trained divisions of the KMT which fought in the battle of Shanghai in 1937, they were also well maintained and very modern; but they only lasted a few weeks against the Japanese army. But the PLA, despite heavy losses, fought the US to a standstill.
Was it due to superior arms? (The PLA were largely dependent on Soviet and old Japanese military equipment and the US had firepower and air superiority)
Larger forces/human wave attacks? (the KMT also fielded much larger armies and suffered heavy casualties without being able to check the Japanese advance)
I think the premise of your question is a bit flawed for a couple of reasons:
The KMT's military and the PVA that invaded North Korea were completely different beasts. The KMT army was a mechanised military that was organised along Soviet lines, whereas the PLA and PVA were mostly light infantry. The KMT army had more in common with the NVA than the PLA.
It's debatable as to whether the PVA was really an effective fighting force. Chinese logistics failed miserably in the face of Allied bombing campaigns. Huge numbers of Chinese troops (either 5 or 6 figures depending on who you ask) died of starvation or exposure during the war, which is quite an achievement when you consider that Korea's climate isn't all that dissimilar to Beijing's or Qingdao's, and is actually milder than Shenyang's or Harbin's. Although they did essentially win back North Korea, they did so at horrendous human cost, with their battle dead outnumbering the coalition's by between 3 and 10 to 1 (again, depending on who you ask)
I talk about this a bit here.