Okay, but here's the real problem.
If the Borg want to assimilate the Feds, why just send one Cube? Why go to all the trouble to time travel a la First Contact? Why try and formulate some kind of plot like Dark Frontier?
All they'd really need to do is gather up a few thousand Cubes, transwarp to Earth (or use a hub) and get busy, honestly. Maybe a few million, if they really want to make sure they'll clean house.
You've got to have a little suspension of disbelief with these guys, both in TNG and VOY.
Most of that is because the borg were not fully formulated and developed by the writers (even though they were) They kept adding abilities and technologies and ended up forgetting what they started out with.