It's absolutely not the same in the eyes of international law.
China-Taiwan is an unfinished civil war. All of the world's countries recognize One China, the only question is whether they recognize Beijing or Taipei as the seat of China's government. Of course, the vast majority of the world's countries, including the West, all recognize Beijing because it is overwhelmingly more powerful.
Ukraine has been an independent country since 1991. Ukraine and Russia recognized each other as independent countries and have respective embassies in each others' country. The world recognizes that there is a Ukraine and there is a Russia, two separate countries.
In addition, there is the question of ethnicity. Even though Ukrainians and Russians are very close, basically Eastern Slavs, they are still considered separate ethnic groups. Of course, there are many ethnic Russians in Ukraine and this is why there are huge separatist movements rising in Ukraine as it takes an anti-Russian course.
However, in Taiwan, the people there are Han Chinese, the exact same ethnic group as the majority of Mainland China. They also speak Mandarin Chinese and even their local dialect, Taiwanese, is basically derived from the same dialect of Southern Fujian province.
So in both legal and ethnic terms, the Taiwan issue is an unresolved civil war.