Yeah. What other ones would there be?
The Empire for Warhammer 40k, but back to this question.
It would be akin to tying up a retarded midget and putting it in a cage match with Godzilla.
No really, it would be murder. The Borg hold thousands of solar systems and millions of Borg vessles. From the weak probe, to the large tactical cubes. Even the weakest of these are a threat to even your average battleship. A Sphere was able to totally beat the pants off Voyager, and Cubes can take on entire fleets of starships. Such as in the Best of Both Worlds, a fleet of 40 starships where easily obliterated, a fleet that could easily destroy 30% of a planet's crust in its opening volly. In First Contact, even after the UFP created more war oriented ships, a single Borg cube was still able to beat its way from a distance nearly equal to that of the Netural Zone, all the way to Sector 001. By the time that the Enterprise E got there, most of the fleet was gone and the Borg cube had sustained heavy damage to its outer hull and had fluctuations in their power grid.
Against an enemy with weaker weapons, far less manuverbility, and next to zilch in the terms of tactics, its going to be murder. Fighters are practically worthless, large ships will be sliced apart and assimilated, and nothing will be able to harm the Borg cube after it adapts.
And before anyone tries the "Borg can only adapt to frequency based weapons" idea, keep in mind that the Borg Cubes can easily block photon torps just as they can with phasers. And you see, photons are matter/antimatter warheads, meaning that their explosive energy doesn't carry a frequency. So, there would have been no way for the Borg to have adapted if all they could adapt to was frequency.
So, the Empire can't damage them save for perhaps ramming, and given the size of the cube and the power of their tractor beams, that isn't likely to happen, and if it does, the ship can repair itself if at least 21% or more of the ship is left intact, or they could beam to another ship and assimilate that, or if the cube and the drones are destroyed, they can just send ten cubes.
In short, the Empire is facing an enemy with greater firepower than even the UFP, numbers high enough that they can't be steam rolled, transwarp hubs that makes their hyperspace look like its standing still, far higher building capacity, the tactical knowledge of every species they've ever assimilated, as well as any tactical knowledge from any imperial officers they assimilate, and the hive mind mentality.