I heard an answer to this on a podcast once,

The deathstar could easily one shot a cube, but they would never be able to do it again, because the borg adapt

If the cube is wiped in a single shot, there might not be enough borg left with enough data to adapt. They aren’t adapting magically, they are able to analyze the shot and adjust their shields to block shots.

Ill bet, of they could sneak up on some cubes they could pop a few before the Borg really got wise.