They might not be able to assimilate 8472, but that doesn't mean that they didn't kill a few billion (trillion?)trying to assimilate them while they were sucking cities into space. that's what the do. they find a planet, suck up all the cities and assimilate the raw resources and civilians. it's just a question of civilian response vs the military response, if 8472 had a military before they met the Borg.
How many cities, how many planets did they excavate, before the Borg were kicked out of Fluidic space?