Interethnic and sectarian conflicts can threaten any large country simply because of its size, and that is why central governments exist to deal with such issues. Another issue is that I don't have strong confidence that a few million inhabitants can so much threaten the integrity of a country with 1.4 billion people predominantly belonging to one ethnic group called "Han", which makes up 93% of the population. For modern China, ideological and property stratification is much more dangerous, when the political system may not keep up with the rapidly growing well-being of citizens who may demand more respect for their own opinions.
China is a very homogenous country, and it gives them a big advantage over other countries such as India, United States and Russia, which are heterogenous. Not just 93% of the population is Han, but a large portion of the population is made of assimilated ethnic groups such as Manchu and Tujia, who speak the Mandarin language and are hard to distinguish from the Han. Only a few smaller ethnic groups such as Uyghur and Tibetan have resisted complete assimilation. And the citizens don't complain much, since their disposable income is increasing every year.
Other countries are caught up in a web of preferential treatment and affirmative action to certain ethnic group, which leads to inter-ethnic hostility.