I think there are a number of differences between Taiwan and Ukraine. Just off hand:
Putin seems out of control. Long time Putin watchers were pretty stunned by his speech’s last week and there are some concerns that he’s unhinged. If Xi starts acting like that, perhaps we’ll have similar concerns with China, but so far, China has seemed like a rational actor — although clearly Xi is moving in the complete dictator direction.
I think it’s far more likely an elderly megalomaniac dictator would just decide “fuck it” I’m going to die pretty soon anyway, I don’t care what happens after I’m gone” (Putin is 69) and decide to launch nukes, in contrast a more committee-based leadership that’s interested in the long-term ascendency of its society is not going to do that.
Putin’s actions last week show either he’s out of touch, or doesn’t care and is just going to roll the dice. Maybe he’s just acting that way, but again, foreign policy people who have been watching Putin for a long time were pretty stunned by his speech and have serious concerns that Putin is losing it.
China’s leadership does not seem to be inclined to use Nukes. (although as noted in the study that could change). That’s in contrast with Soviet era policies for both NATO and USSR which suggested things were far more likely to spiral into a nuclear exchange.
Taiwan can be a more self-contained theater than Ukraine. Taiwan is obviously an island. Attacks can be limited to naval targets, air targets in Taiwan’s airspace, and ground targets actually in Taiwan. This just isn’t the case with Ukraine. There is way too much opportunity for spillover.
Russia has a lot more opportunity to play tit-for-tat with low-level responses because of geography. For example, shelling Poland from Belarus or one of the Baltic States with artillery could be used as a light “punishment” that would be in a bit of a grey zone for how to respond. For example, they could fire a few salvos of artillery at NATO countries every time a Russian fighter is downed. This could lead to a no-fly zone over Ukraine paradoxically proving that NATO is in fact a paper tiger if we don’t respond to direct shelling of a NATO country. China could launch missiles at Japan, but that would be a lot more provocative. Also we would at least have the appearance of attempting to protect Japan with anti-missile technology such as Patriot batteries — how effective they would be is an open question, but even appearing to take action makes a difference here. There is nothing we can do to prevent artillery fire into NATO countries other than direct retaliation. China has fewer lower-level retaliation options. Russia has a lot of them.
Russia has much better technology, particularly with respect to their anti-aircraft systems, which are just top notch. There are a lot of concerns that without taking out Russian anti-aircraft located on Russian soil, a no-fly zone is completely undoable. A US President would be facing a lot of pressure to attack anti-air and anti-ship systems located on Chinese mainland, but the possibility of carrying out some operations without doing that are far more likely.
Russia has a lot, lot, lot more nukes than China. Although China is building more.
