The Borg Queen states openly while interrogating Data that she considers the Borg's biggest weakness to be their remaining vestiges of humanity (humanoidity?), something that the Borg constantly strive to "transcend" as they assimilate and adapt (in contrast to Data, a sentient artifice who seeks to become more human).
The Borg are a major threat in Star Trek: TNG and Star Trek: Voyager (I haven't watched the latter, but looking at this it seems they are). Those two series happen at roughly the same time as DS9...
star trek - Why is there no Borg threat in the DS9 series? - Science ...
The Borg tend to justify their actions by claiming that they are "improving" species, bringing them closer to perfection. They are bringing order to chaos. Basically they reckon they're the good gu...
Additionally, if the Borg treated the Federation and related Alpha/Beta powers like they did species 8472 and sent hundreds of cubes instead of just 1 each time, this answer likely wouldn't exist. It's less that the Federation was successful and more that the Borg didn't respect the Federation seriously enough.
star trek - What are the known Borg failed assimilations? - Science ...
So the Borg are introduced to the Federation by Q, but the fact remains the Federation lives on the other side of the Galaxy, and aside from more possible encounters with Q / Caretaker like beings,...
Why do the Borg care so much about Earth? - Science Fiction & Fantasy ...
In Regeneration Starfleet learns about a culture that we (as audience) know as The Borg. According to TNG, the Federation doesn't seem to know anything about these beings when they "first" meet them.