I think the general consensus is that it would be pretty awful but not as bad as Mao through until the 1970s.
I'd say it would be just as bad as Máo, but for different reasons - less the Great Leap Forward and more of the Cultural Revolution-era Máo. My main suspicion, because of how the GMD under Sun thought it would be a good idea to copy the Soviet model of government but in a model appropriate to China's circumstances, is that the more of the Mainland the ROC retains during the Civil War, the more it begins to resemble the PRC, with the GMD in the role of the CCP. Yet even that does not begin to capture the essence of the GMD during the Nanjing decade, where the Nationalist regime was just as bad as the Beiyang government in having direct control over part of China and in its factionalism, which the GMD would want to re-emulate. Among other things, though not as obviously fascist-looking as before, the
New Life Movement could come back with a vengeance as a way of cementing Jiang's control, and it is here that I'd find my Cultural Revolution analog as a way to prevent the young from becoming attracted to Communist temptation. Furthermore, a ROC that wins the Chinese Civil War would have a triumphalist streak (probably bordering on arrogance) that would basically lead the Party to believe that doing nothing to resolve the underlying causes as to why people were discontent with GMD rule. I'd expect more of the same of the purges and trying to eliminate any challenges to Jiang's rule. In that case, Jiang would ITTL basically be a variant of a Qing dyansty Emperor until his death, with Jiang Jingguo as the obvious successor in this case. (But of course how would the head of the secret police try to justify his claim of being President of the Republic of China?) The 1947 Constitution - the period of "constitutional government" under the GMD's ideology - would just be a figleaf for a de facto dictatorship similar to Mexico's 1917 Constitution and the reality of the bad old days of the PRI, or the dictatorships of Salazar or early Franco, or for that matter something similar to Pakistan considering how Jiang's real power base was in the Army. It would be justified with the doctrine of
dǎnnguó, or the party-state, which could define everything including its particular
its particular brand of socialism and its harassment of Shanghai merchants.
As a corollary to this line of thinking is my response to this post:
It would be interesting how the Kuomintang deals with Macao and Hong Kong.
Since the ROC was similar to the PRC in denouncing the unequal treaties and demanding that all former colonies eventually come back to Nanjing's control, as well as an extension of the elimination of any challenge to Jiang's rule, Macao and Hong Kong would be doomed. Of the two, HK would be the top prize, considering that it was the territory itself that was emblematic of the Century of Humiliation as a result of the Opium Wars. Retaking HK thus would be a top priority of a triumphalist ROC, damn the consequences - and no special status for Hong Kong for you, with any resistance to GMD rule treated much like how the GMD treated any other challenges to its authority (with Taiwan's White Terror instructive in this case, up to and including suppressing Cantonese). Macao, OTOH, is a different kettle of fish - since Macao in the grand scheme of things was a backwater after HK's ascendance (there's a reason why Macao is better known for its gambling), and is under a similar dictatorship under Salazar, it might get away with more than Hong Kong. In that case, more of Macao's "Meditterasian" culture, up to and including Patuá (aka the Macanese language) and the territory's particular version of the Guangzhou dialect of Cantonese (with minor differences in pronunciation and vocabulary, primarily from Portuguese and Malay), could be retained. As a result, Macao's handover to Nanjing's rule, similar to OTL, would be smoother since Macao - despite also being a major beneficiary of unequal treaties (despite existing prior to them existing) and hence a target for reclamation - is not as much a perceived "threat", though after the Revolução dos Cravos (Carnation Revolution) it might be seen as such (unless Salazar finds a way, à la Franco, to bring back the Portuguese monarchy as a continuation of the Estado Novo - though even then, much like Juan Carlos in Spain, that would probably not be the actual result - which would mean the Carnation Revolution would not exist in the first place).
Hence, it would ultimately in the 1970s and 1980s that things could change, but more like how democratization on Taiwan IOTL (and, if he was correct, what Zhao Ziyang's ideas were for the Mainland) was supposed to play out, which would be basically Singapore writ large, with a heavy dose of "Asian values" rhetoric, hence the GMD as the party who would win election every time. An opposition would be permitted to exist, but it would be marginal and unable to mount any effective challenge to GMD rule (though not without attempting to try), at least during the 1980s and 1990s - the 1997-1998 Asian financial crisis would be the force that creates a genuine democratic opening, or one of the first opportunities to do so, so it would only be in this century ITTL that the GMD would finally be forced to cede power to a democratic opposition (and without HK as a safety valve for all that pressure, it would have to be in this manner since the GMD basically would monopolize all access to power - you could almost take OTL criticisms of Chinese Communist rule, change some minor details, and pass it off as a carbon copy of TTL's criticism of Nationalist Chinese rule). China as a whole would recover, but it would take a very long time to unpack the GMD's long shadow and legacy.
You're not likely to see an economic boom like Japan at least until then, and there's a strong risk that China just ends up a semi-industrial country like India after that rather than getting into the big money. For instance, I can imagine that if the Americans sunk their teeth into the Chinese economy like they were always planning to (the Open Door Policy reaches its end-state if the Nationalists win on the back of US support) you could have English getting popular in China way earlier and there being loads of Chinese call centres instead of Indian ones. If Chiang doesn't get his act together, he may end up reliant on Western backing and investment to hold on, leaving China a peripheral power come the 21st century primarily preoccupied with Indian (and Russian) rivalries.
Possible, but I don't see English becoming that popular in China ITTL, even with US backing.
Then there's the idea, which would make a pretty interesting scenario, in which Nationalist China basically takes over the nonaligned movement and plays the US and USSR off each other while building its power via a mixed economic model which is more or less a precursor to OTL Chinese state capitalism.
That definitely makes sense; the ROC under Jiang before the move to Taiwan was not really all that enamored with the US, so this would be something totally within Jiang's character.
In this scenario, you might see the Viet Minh getting Chinese backing anyway, just to insulate Indochina from foreign influence while making America really uncomfortable.
In that case, the Việt Minh would be forced to merge with the VNQDD - the GMD's satellite party in Vietnam - as a way of neutralizing Communist influence. The GMD's influence would even reach
as far south as Malaysia.
Think an ideologically flipped version of Yugoslavia, right down its baptism of fire in a homegrown resistance movement and a general running the show. It might even collapse in an especially grisly style once Chiang dies.
In any case, Kim Il-Sung probably isn't sticking around for long.
Makes sense.