Nanotechnology & Knowledge from countless civilizations

Borg's nanotechnology is so advanced than it can modify warp drives to make them more powerful within few hours.

From Wikipedia:

Nanoprobes are microscopic machines that inhabit a Borg's body, bloodstream, and many cybernetic implants. The probes maintain the Borg cybernetic systems and repair damage to the organic parts of a Borg. They generate new technology inside a Borg when needed and protect them from many forms of disease. Borg nanoprobes, each about the size of a human red blood cell, travel through the victim's bloodstream and attach to individual cells. The nanoprobes rewrite the cellular DNA, altering the victim's biochemistry, and eventually form larger, more complicated structures and networks within the body, like electrical pathways, processing and data-storage nodes, and ultimately prosthetic devices that spring forth from the skin.

Also, it's a well known fact that Borg only assimilates sufficiently advanced civilization which makes sure that it gets new knowledge (like time travel).

We are the Borg. Lower your shields and surrender your ships. We will add your biological and technological distinctiveness to our own. Your culture will adapt to service us. Resistance is futile.

(Star Trek: First Contact)

Combine these two and you've a formidable weapon.

P.S. Borg's adaption and assimilation capabilities get better over time. For example, Borg got nanoprobe technology after assimilating Species 139.

Per the TNG movie ST: First Contact, we see the Borg use Nanoprobes to assimilate people. The series never dealt with the particulars, but the movie makes it clear they inject people with them, which makes them part of the collective shortly thereafter (it happens to several members of the crew in the movie).

Once a person has been assimilated, their knowledge is added to the collective. From The Best of Both Worlds Part II

RIKER: They couldn't have adapted that quickly.
PICARD [on viewscreen]: The knowledge and experience of the human Picard is part of us now. It has prepared us for all possible courses of action. Your resistance is hopeless, Number One.

They also have strength in numbers. We see in the same episode that they lose several drones before adapting. The dying drones transmitted data to the collective, allowing them to devise a defense. With thousands of drones, you can afford to brute force the problem by throwing drones at it until something works.

It would be cumbersome to be the Borg equivalent of a Swiss Army knife, with an attachment useful for any possible act of defense. Instead, it is more efficient to have the information for building any possible defense in short order. The Borg have assimilated the technology of many species, and have the knowledge ready to use. Clearly, their technology includes the means of replicating new items within seconds to minutes of needing them.