What was subtle about "Dark Frontier"?
Using a virus to assimilate Earth instyead of beaming down lots of drones and using hand tubes to assimilate people. It would take months though.
QUEEN: Our previous attempts to assimilate them were all direct assaults. They failed. So we've created a more surreptitious strategy.
SEVEN: You intend to detonate a biogenic charge in Earth's atmosphere.
QUEEN: It would infect all life forms with nanoprobe viruses. Assimilation would be gradual. By the time they realised what was happening, half their population would be drones.
Definitely more subtle than the Borg's usual strategy.