Russia wasn't that multi-ethnic in nature. The Slavic Russians continue to be the overwhelming majority and the dominant ethnic group in almost every part of the modern Russian Federation, and they're still a large and significant minority in most of the former Soviet Union states. Qing China, similarly, was not that mono-ethnic, especially in the outlying regions of Xinjiang, Tibet, Yunnan, and (now independent) Outer Mongolia.
Being too lazy to find a primary source, according to Wikipedia as of 1897 out of 125 million people in the Russian Empire, the primary language spoken was
Russian 55 million (less than 50% of the population)
Ukranian 22 million
Turkic-Tatar 13 million
Polish 7.9 million
Belarussian 5.8m
Yiddish 5 million
Finnik 3.5 million
Modern countries that were in the empire include Finland and Poland, in addition to the now ex Soviet Republics.
Only 70% was Eastern Orthodox, among Roman Catholics, Muslims, Jews, and several other groups.
Russian Empire was about as diverse as it gets. Even if it can make the appropriate political reforms so that the Czar remains in power, I find it unlikely the empire lasts as eventually those ethnic groups will want to be on their own. I guess at that point it is semantics then as to what constitutes an empire.
I cant speak to the Qing politically but the fact that China retains fairly similar borders despite the chaos of the 20th century says something about the cultural dynamics even if I overstated its homogeneity in my first post. If the ethnic diversity were truly that distinct, the probability of it splitting up between 1912 and 1950 would have been pretty high.