TL;DR: The Republic of China was so corrupt that for most of its history, the central government in Nanjing had little or no authority over certain areas of China, such as in Guangxi (where the famous Guanxi Clique was based). Many local politicians did whatever they wanted in a climate of corruption that was exacerbated by the Japanese invasion in 1937, where the Communists were seen in many areas as the only ones doing any actual real fighting against the Japanese.

So I'll start off by making a note that pretty much the entire existence of the Republic of China after Sun Yat-sen's death, which is very early, was a power struggle between various politicians such as Wang Jingwei, Hu Hanmin, Sun Ke (Sun Yat-sen's son) and of course Chiang Kai-shek himself. This tenuous relationship between ambitious and greedy politicians forced Chiang to organize certain political groups such as the Blue Shirt Society, and tie himself into other politically-involved, rich families. The family he chose to do this, perhaps because of love but certainly because of money, was the Soong family, and TV Soong would find himself as Minister of Finance in the 1930s and Premier of China after WW2. The constant battle for supremacy over China was so fierce, that TWICE Chiang purposely stepped down from office, "retiring" from politics. Furthermore, in 1936 Chiang was actually kidnapped by his friend Zhang Xueliang at Xi'an (the Xi'an Incident) for a few days before he safely returned to Nanjing.

So with that out of the way, lets try to hit every area possible for just how corrupt the KMT was for its entire existence on the mainland (this corruption continued in Taiwan, however since your question only asks up to 1949, I'll stop there).

Jay Taylor often points out in his grand biography of Chiang Kai-shek, The Generalissimo, how Chiang lamented of the corruption and opposition he faced as leader of China. Chiang it would seem, although a Christian convert, stuck to his Confucian roots, and many of his passages remind the reader of a proper Confucian complaint. Corruption is certainly one of the gravest issues Confucianism attempts to face, so its apt that he had such strong feelings. Taylor points out that Chiang himself was not necessarily "corrupt" in the sense that he wanted to levy high taxes on the peasantry and just cause general havoc, but that rather Chiang was often forced into these positions due to the instability of China throughout the period. That being said, it was well known that Chiang allied himself with various gangster bosses, particularly the Green Gang and their leader Du Yuesheng.

Chiang used the Green Gang to often engage in political purposes such as breaking up labor strikes and protests, but also as a tool to extort the local rich population into paying taxes, perhaps by kidnapping one of their oldest sons. Throughout the 20-30s, the Green Gang played a prominent role in assassinating members of the CCP, all the while continuing to sell opium. TV Soong, Chiang's aforementioned brother in law and minister of finance, used the Gang to pressure Shanghai banks into keeping interest rates law and buying national securities.

The Guangxi clique is one of the few infamous rival groups who basically ran an autonomous nation in southern China for the majority of the 1930-40s. This group was led by Li Zongren and Bai Chongxi, two generals who would become famous in WW2 for their competence and tenacity in the face of the Japanese invasion. Although they worked often with Chiang, especially during the war, their running of a separate authoritarian warlord state was seen by many intellectual Chinese as the proven weakness of the KMT. It is reasons like this which helped lead to many intellectuals fall in line with the CCP after WW2.

While letting local warlords and gang bosses run KMT-owned cities and provinces, Chiang got himself absorbed in his controversial fascination of destroying the CCP. High taxes, banditry, a failing currency (most Chinese at this point used Mexican silver dollars because of their value) was all leading to bad news that would explode after the War. Many intellectuals were disturbed at what they saw as negligence of the part of the KMT for focusing solely on the destruction of communism. At one point even before WW2, the KMT's budget for military spending was as high as 70% of the total budget, an astonishing amount. Making matters worse, in 1931 Japan annexed Manchuria, which was met with no real response from Nanjing.

So now its 1937, and all the aforementioned problems are plaguing China. A formal invasion of China did have the effect of practically uniting most warlords under the KMT, but the CCP still ran as a separate entity in Yan'an. Remember that for the first four years, China was basically alone in its war against the far-superior Japanese, and by 1942 the KMT had lost upward of three million troops. The majority of eastern China is absolutely destroyed due to war and brutal scorched earth tactics carried out by both sides (Chiang actually ordered the destruction of all the dykes on the Yellow River). To exacerbate this, multiple loans for billions of dollars had been taken out by the KMT from the US after 1941, and Wang Jingwei, the once powerful Centrist/CCP member, defected to the Japanese as China's most famous "collaborator." Coming out of the war, there were some serious issues to be fixed.

But, the fixing was just not to be done. Chiang and Mao, with both their eyes on the prize, were ready to stay in control/seize control, respectively. Peace talks broke down, TV Soong resigned as head of finance (there were allegations he had been siphoning money from the treasury for retirement), and China's currency was effectively useless. The military budget continued to be stuck at around 70-80%, but the KMT still couldn't afford even enough ammunition to defend itself, while the CCP enjoyed lavish loans and materiel aid from the USSR. The rest is well recorded in history. Peng Dehui and Lin Biao seized Manchuria, and from there the flood of communism was uncontrollable as Chiang and whatever loyalists/anti-CCP members were left, fled to Taiwan.

Conclusion: As I said, its important to understand that the KMT really had little to no authority over China outside of Nanjing. Local warlords and gangs levied harsh taxes on peasant populations, which was occasionally sanctioned by KMT officials, allowed banditry to go unchecked, and just generally did a poor job of tending to the economy. This all caught up to them as the civil war came to a close, and in the eyes of the rest of the world, especially the UK and US, the KMT was not trustable; the lesser of two evils but even worse they owed the US a lot of money where as the CCP did not.

Chiang may have been a much more effective leader in a period of stability and peace; and there is some evidence of that in Taiwan, where the 60-80s saw a booming economy. Chiang died in 1975, 87 years old, so much of that initial growth was under his, and then his sons, guidance. But the times were tough for the young republic, and it ultimately led to a China filled with corruption.

Sources:

Jay Taylor. The Generalissimo: Chiang Kai-shek and the Struggle for Modern China.

Graham Hutchings. A Province at War: Guangxi during the Sino-Japanese Conflict, 1937-45.

Yin Ching-yao. The Bitter Struggle between the KMT and the CCP in Asian Survey vol. 21, No. 6 (Jun., 1981).

https://china-journal.org/2018/02/26/why-did-chiang-kai-shek-lose-china-the-guomindang-regime-and-the-victory-of-the-chinese-communist-party/