SoA2 Borg are powerful by design, and intention. Some have a lot of trouble dealing with them. While others may have no trouble at all. Knowing your enemy, and yourself, Strategy, and a little luck have a lot to do with it. We designed the SoA2 borg to where if one single faction fought them they would be very VERY difficult to defeat.
The developer intention was to make the Borg to where you would almost need all four of the other factions to fight them. Single player game could consist of you playing as the feds. Along with AI, or Human controlled Klingons, Romulan, and Dominion. The Borg would be a locked AI faction, because it is all but impossible to form diplomatic relations with the Borg. As the game progresses you would skirmish with all factions until you built up enough diplomatic points to form alliances. The Feds have the advantage in Diplomacy. You may, or may not be able to form alliances with all four factions.
By this point in time all should be teched up enough for the "real" fighting to begin. Playing vs AI Borg is like playing "beat the clock". The more you allow them to tech up. The harder it will be to beat them. If they get cubes on the map you will need all the help you can get. If the Borg by small chance manage to max out their research, then you, and your allies deserve to get your ass's handed to you. The Borg do not research like the rest of the factions. Their labs cost more in both resources, and logistics. This was by design to make them research more slowly than other factions. To show how "methodical" they can be. This will allow you to tech up enough to where you can do some damage to the Borg, and slow them down while you make allies that can help you defeat them.
While you can do it. Playing as Borg is not intended. The Borg are meant as an AI faction, but by the nature of Sins we had to make them a playable race. I usually play as Borg just for fun to clear out the map in a ridiculously short time, and to see how AI versions of the other factions play out.