I bet they weren't as long they were in Poland or Lithuania or whatever.
They weren't what? Being the beneficiaries? In the Poland, no, they were not. In the Grand Duchy of Lithuania, they actually were, because they constituted the majority of the population after being conquered, and in a matter of decades became to dominate, to the point where GDL's code of laws would be written in a dialect of East Slavic that'll later become Ukrainian and Belarusian; and Slavic elite - boyars, wealthy landowners etc - merged into (and largely overwhelmed) the Lithuanian one in the state.
But I simply don't see how they would be any different than any other territories in either Russian Empire or Soviet Union. Both of those states were to large degree nation agnostic.
Seriously? You'd have to be Russian to claim that, since it was only nation agnostic for that one particular nation.
Russian Empire was explicitly Ukrainophobic, to the point of denying the existence of Ukrainians as a separate nation with their own language outright, all while actively trying to eradicate it. Heck, Ukraine was already feeling oppressed a few decades after signing Pereyaslav Rada.
Soviets under Lenin initially reversed that process and encouraged nation-building (so long as the economic system would remain communist). But Stalin quickly put an end to that, with Ukrainian language and writing system being "depolonized" (really, russified), Ukrainian intellectual elites being persecuted, and Ukrainian churches not associated with Russia (i.e. anything other than Russian Orthodox Church - such as Greek Catholic uniates and Ukrainian Autocephalous Orthodox Church) closed down.