Russification is certainly possible. Stalin himself dabbled in linguistics and had he decided that Belorussian and Ukrainian were "low" or "primitive" forms of Russian, and had his legacy survived his death, it may have continued. The "problem" is that the Soviet ideology lacked the requisite Slav nationalist component until World War II and after that was not played up to the right volume, nor was it ever institutionalized.
In China, the centralized government forced what was essentially Beijing Mandarin upon millions of educated and literate people in the southern and eastern parts of the country, places where the local spoken language was maybe 10 percent intelligible with the declared standard--far more divisive than Ukrainian and Russian. Widespread proficiency in a form of Mandarin that actually matches this standard is a relatively recent phenomenon, created by compulsory education and media infrastructure. Along with this, Chinese ethnic policy created (reinforced) a Han race out of a group of people as large and arguably as diverse as the population of Europe.