In theory the Borg would be nearly unbeatable.
Think for a moment how originally the Dominion weapons went straight through federation shields (and they took out a galaxy class starship). What then? Well the federation pooled its minds together and all those brains working together combined knowledge, experience, and data to come up with a way to adjust their shields so that when the Dominion tried to take DS9, they found the federation shields holding. "I've learned to never underestimate the Federation's technical skill. Or Captain Sisko's resourcefulness." -Dukat.
The Borg are essentially all that - super streamlined. Instead of needing months to gather the nerds together and get the committees working, as soon as they are hit with a weapon, the nerd specialists in the collective are pouring over the data and developing the countermeasures in minutes, instead of months.
The original BoBW ended the only way a fight with the Borg ever could. By using their interconnectedness against them and sending a bad command into the hive mind that got them shut down.
It really is ironic when you think about it. Humanity's super power that has let them conquer Earth is basically cooperative teamwork. The Borg is the ultimate manifestation of that super power. If something like them were encountered out there in space - could you ever really beat them?
It's no wonder the writers started gimping them.