The trek weapons only seem weak because of their advanced shielding

Not to mention the long range scans would show the source of the deathstars shield to be external

Easy pickings. That and even if the laser were effective, the borg cube would never maneuver in front of it

Its kind of obvious where it comes from by sight, not to mention long range scans. I doubt the deathstar could pull a quick 180deg spin and shoot down the cube
oops missed the post about no time for long range scans

Well from the movies its apparent that it takes a while for the deathstar to fire

short range scans would give them enough information to avoid fire.