Aside from the obvious "It could still trigger a war with the US" despite any US desire to not get involved, there are quite a few potential repercussions independent of US involvement should China invade and conquer Taiwan. Those don't exist, or don't exist nearly as strongly, in the Russia-Ukraine relationship/war.
First, an actual Chinese invasion of Taiwan would demonstrate to China's neighbors that they, too, could very well be the target of Chinese military aggression. The only reason Japan is not a significant nuclear power on par with China is because Japan doesn't want to be that. That could very well change should China invade Taiwan. A nuclear-armed Japan with a nuclear arsenal comparable to China's would seriously crimp China's ability to dominate east Asia. And if Japan turns the switch on a nuclear weapons program, how far behind would South Korea be?
Second, countries such as Vietnam and Philippines that have claims in the South China Sea that conflict with China's would be a lot more wary of China - Vietnam has even fought a war with China in 1979, so a Chinese invasion of Taiwan would rekindle fears of continued Chinese aggression. Note that one of the reasons China gave for attacking Vietnam in 1979 was Vietnam's then-control of the Spratly Islands, which China claims.
Note that if the US failed to act militarily to support Taiwan should China invade and likely conquer it, nations such as Japan, South Korea, Vietnam, Phillipines, and others would certainly feel even more need to take a much larger role in defending themselves from China.
Third, there's little economic gain - Taiwan's economy, although about the 20th largest in the world, is but 4% of the Chinese economy, and the Taiwanese economy is heavily dependent upon exports to nations that would likely cease buying from Taiwanese sources after a Chinese invasion, driving that 4% even lower.
Fourth, what happens to China if the world stops buying their exports? Economic sanctions against China limiting exports could potentially be absolutely devastating to the Chinese economy.
Fifth, countries could send all the Chinese students in universities home. Or seize property owned by Chinese companies and individuals. Such actions would disproportionately impact rich and powerful Chinese - and their entire families. All those high muckity-mucks in the CCP who finally got to send their annoying, do-nothing, lazy teenagers abroad and enjoy the fruits of their long labors in peace would have to deal with boomerang college students. That may be a bit of a humorous take, but the internal dynamics of CCP senior members likely plays a huge role in Chinese national decisions.
In short, a Chinese invasion of Taiwan would be an expensive operation that could provoke a serious war with the US, compel neighboring powers Japan and South Korea to obtain nuclear weapons, cause China to lose all trust with other east Asian countries such as Vietnam and Philippines, and if successful would likely destroy a significant portion of Taiwan's economy, making the acquisition much less valuable. And erode support for and from senior CCP officials.
None of those factors apply as greatly to Ukraine. Poland is almost certainly not going to try to obtain nuclear weapons to deter Russia, Ukraine doesn't have anywhere near as much international commerce as Taiwan, Russia only exports natural gas and oil, which as commodities are going up significantly in price as sanctions on Russian exports are put in place - which means there will be a lot of pressure soon to relax those sanctions (similar to how France moved to stop sanctioning Saddam Hussein's Iraq not long after the 1991 war in exchange for rights to Iraqi oil). How long will Germany want to punish Russia if the cost of natural gas in Germany is three or four times more than it would be without sanctions on Russia? Putin certainly knows this - sanctions will almost certainly be short-lived.
And if Russian oligarchs didn't support Putin's invasion, it likely wouldn't have happened. Or maybe Russia would have new oligarchs who did support the invasion...
Finally, the main difference.
Russia views Ukraine as a buffer against NATO, and they also view the 2014 US-led deposing of Russia-friendly Ukrainian President Yanukovych as an illegal coup. With a literal "Fuck the EU" attitude*, the US poked Putin's Russian bear right in the eye in 2014. And just in the past few months, the Biden Administration support for Ukrainian admission to NATO only poured fuel on that fire. Imagine if Russia had deposed the government of Mexico, installed a regime friendly to Russia, and then a few years later signed a treaty with that Russia-friendly government to station Russian weapons and troops in Mexico?
That's what the US did with Ukraine, and that certainly does not apply to Taiwan.
And yes, that is what the US did. BBC commentary:
Jonathan Marcus: The US says that it is working with all sides in the crisis to reach a peaceful solution, noting that "ultimately it is up to the Ukrainian people to decide their future". However this transcript suggests that the US has very clear ideas about what the outcome should be and is striving to achieve these goals. Russian spokesmen have insisted that the US is meddling in Ukraine's affairs - no more than Moscow, the cynic might say - but Washington clearly has its own game-plan. The clear purpose in leaking this conversation is to embarrass Washington and for audiences susceptible to Moscow's message to portray the US as interfering in Ukraine's domestic affairs.
Umm, the US clearly was "interfering in Ukraine's domestic affairs", despite the EU knowing Russia considered such acts extremely provocative:
Jonathan Marcus: Not for the first time in an international crisis, the US expresses frustration at the EU's efforts. Washington and Brussels have not been completely in step during the Ukraine crisis. The EU is divided and to some extent hesitant about picking a fight with Moscow. It certainly cannot win a short-term battle for Ukraine's affections with Moscow - it just does not have the cash inducements available. The EU has sought to play a longer game; banking on its attraction over time. But the US clearly is determined to take a much more activist role.
The US clearly "pick[ed] a fight with Moscow", one that the EU did not want to engage in. Hence Nuland's "Fuck the EU" comment.
Jonathan Marcus: Overall this is a damaging episode between Washington and Moscow. Nobody really emerges with any credit. The US is clearly much more involved in trying to broker a deal in Ukraine than it publicly lets on. There is some embarrassment too for the Americans given the ease with which their communications were hacked. But is the interception and leaking of communications really the way Russia wants to conduct its foreign policy ? Goodness - after Wikileaks, Edward Snowden and the like could the Russian government be joining the radical apostles of open government? I doubt it. Though given some of the comments from Vladimir Putin's adviser on Ukraine Sergei Glazyev - for example his interview with the Kommersant-Ukraine newspaper the other day - you don't need your own listening station to be clear about Russia's intentions. Russia he said "must interfere in Ukraine" and the authorities there should use force against the demonstrators.
* - I seem to remember the reason the EU and especially Germany didn't support the US-led ouster of Yanukovych was because they viewed that act as something Russia would view as an act of war, but I can't find documentation for that. That view seems prescient, and if Russia viewed deposing Yanukovych as a less-serious act of war, you can imagine what they'd think of Ukraine joining NATO.
Note also that the expletive is a direct quote from senior US State Department official Victoria Nuland. IMO it's absolutely critical to capturing the US attitude towards interfering in Ukraine right on Russia's doorstep and therefore important in understanding differences between Russia-Ukraine and China-Taiwan.
