Star Trek: Every Borg Ship Explained

Although even the smallest Borg vessels are heavily armed and shielded, Tactical Cubes are the pinnacle of the Collective's military might.

They're outer surface is much more protected than a standard Cube, with armour plates covering a large amount of the hull. In addition to this armour, the central plexus (the device that connects the drones aboard the Cube to the rest of the hive mind) was protected with a special kind of shield known as a multi-regenerative security field.

A Tactical Cube was only seen in one episode of Voyager, Unimatrix Zero. In the episode, Voyager was clearly outgunned by the Cube, and they were only able to defeat it by forcing the Borg Queen to activate its self-destruct.

A single ordinary Cube is usually enough to assimilate a whole species, so Tactical Cubes, with their vastly superior firepower, are likely only used against species who put up significant resistance.