I always saw a singular flaw with the Borg. Despite the different Borg Queen's rationalizations given to Picard, Data and Janeway, the entire concept of the species was 'illogical'. That comes in by what Thaddeus brought up as the Borg primary objective, 'absorbing species which display useful technological skills and advancement.
When absorbing a species and their potential is rendered inert thus causing the Borg to constantly search for new species advanced or advancing to a level deemed useful. Though life seems prevalent in the STU, advanced interstellar cultures are rare.
Though it is unclear how old the Borg as a species are since their origins have never been discussed, pillaging and absorbing cultures the way they do is impractical. Everything written about the Borg suggests they are highly logical on an order superior to Vulcans. Any species with that level of logic based on a cybernetic culture would recognize their method of acquiring 'new blood' was inefficient. Though the Borg are highly adaptable, they cannot innovate which is why the Federation has been able to thwart them time after time.
I also found that Species 8472 being unable to overcome the Borg was also illogical. It was clear from their introduction they had the tactical and technical advantage over the Borg. They also proved to be innovative by creating that training center whereby they copied San Francisco, Starfleet Academy, Starfleet HQ and took human form to learn more about the 'human experience' in order to infiltrate Earth.
The Borg would never have wasted time or resources on something so elaborate. Even with their Transwarp Conduits, the Galaxy is unbelievably vast and they could go hundreds of years without finding suitable species to absorb. At least ants know to cultivate 'herds' of livestock (aphids) or crops (fungi) to sustain their colonies. The Borg seem much too simple and opportunistic to be so advanced a species capable of long-term viability.
