A key element to look at here is that Mao was able to very successfully rewrite history after his ascension to power and to pull a great big hoodwink on the relative power of the NRA relative to the PLA. The GMD and its National Revolutionary Army was the most powerful force, due to aid from, in a sharp irony, the Comintern and the help of the emerging singular dictator of the USSR Josef Stalin. The USSR's role in the emergence of the GMD and in the Northern Expedition is incapable of being fully severed from it, as it gave Jiang a good portion of his early weaponry and military doctrine. What the USSR didn't grasp about Jiang, or ultimately Mao, was that he was a Chinese nationalist and not prone to being beholden to any foreigners who could have a squeeze on him.

The first phase of the Chinese Civil War was far more lopsided than the second because the CCP, not yet fully led by Mao as the singular feature, was far smaller and weaker as a movement and Jiang was barely stopped from outright throttling it to death in the sieges of the early Soviet Republic in the mid-30s. Jiang did have major defects, but that initial foreign aid and his careful co-option of the warlords he didn't land on both feet gave him an insuperable advantage in firepower and bodies over anyone that tried to oppose him then.

Then a little something called the Second Sino-Japanese War landed on his armies with both feet shod in concrete from fifty feet up. Mao spent most of the war consolidating his power in the Shaanxi Soviet, hoarding his strength and fighting Japan all of once, in 1942 (which ended spectacularly badly for him with the Three Alls Campaign as the punitive expedition in response. The Alls in question? Kill All, Burn All, Loot All). Jiang was committed to a much harder war with Japan from the first battle at Shanghai through to 1945. For most of this war Japan had the same firepower and mobility advantages relative to Chinese forces that Soviet and Western armies had relative to it in the Pacific War and Mongolian-Manchurian campaigns.

This advantage told very heavily in its favor, starting with Shanghai and the first phase of the campaign, and the ability of the Japanese to go anywhere they wanted whenever they really wanted to go there. Japan could not, however, occupy all of China and it met several local defeats, one city, Changsha, holding out against them for three Herculean battles before finally falling in the fourth.

Beyond all this, there was the third great Chinese figure of the war, Jiang's old confederate, Wang Jing-Wei, the main Chinese collaborator with the Axis (and the last Qing Emperor as Emperor of Manchukuo, though that's a separate topic to a great degree). Wang had limited real power and influence, especially after the Rape of Nanjing, but he reflected a reality of a belated post-Nanjing Massacre attempt at divide and conquer methods that did not pan out very well for obvious reasons but still existed, and so did its impact.

Additionally Jiang, after all these years of brutal warfare virtually in isolation was saddled with a British Empire that wanted a weak China to loot indefinitely, and a USA that put him on a pedestal and vastly simultaneously invented phantom strengths that did not exist, and demanded Jiang fight for US goals, not his own, and refused to understand much of what actually led Jiang and his armies in strategic priorities. Jiang ultimately won a hollow victory against General Stilwell as a result of this, one of the main victories he won in WWII with allies, but it left his armies saddled with a reputation of weakness and refusal to fight that wasn't really justified by the facts.

In 1944, Japan won its last great strategic victory of WWII in Operation Ichi-Go, where Jiang's army collapsed in morale terms and his armies became enfeebled relative to their opponents to a point that it began to become possible for Mao to beat them wholesale in a renewed conflict.

August Storm, the Soviet invasion of Manchuria, broke the Kwangtung Army, which due to depletion in Pacific War battles was enfeebled, even if it was powerful enough to oversee Operation Ichi-Go, and left both Soviet looting (and mass rape) and the USSR (plus to a lesser degree Imperial Japan) giving Mao sudden access to firepower and lavish weaponry that he never had before, nor skill in using them.

After WWII, Jiang, who spent much of the 1942-5 period stuck fighting the Japanese in Burma with the USA, due to the severity of the defeats up to 1942 and further amplified by the Ichi-Go Offensive, was required to move his troops over the better part of a subcontinent to the north, in a devastated country that is the only WWII society other than the USSR to rival it for the immensity of losses and how much of it was rubble after the fact.

He was at least partially encouraged by US advisors to do this, and when the realities of logistics and wartime devastation told, unfairly blamed for factors outside his control. The ultimate defeat in the three campaigns of the post-WWII era was the simple matters of logistical overstretch, exhaustion from a war his armies had done so much to fight and Mao's armies so very little to fight, underestimating the ability of the PLA to shift from securing control of the Shaanxi Soviet to fighting major overland campaigns with conventional massed armies, and inability to recoup the losses of the Second Northern Expedition in time to repair the damage.

The defeat in the final phase of the Chinese Civil War owed much more to World War II and the genocidal rampage of the Imperial Japanese Army than it did anything to some innate Communist skill at arms.