The Borg ship in "Descent" WAS a real Borg ship. It wasn't some patchwork thing, it was an authentic Borg ship.
The Borg Tactical Cube was smaller and weaker than an Assimilation Cube, the name alone (Tactical, not strategic) suggests that its' not meant for Civilization Assimilating like the Assimilation Cubes are.
Also, VOY was more advanced than the ENT-D was (it was a post-Wolf 359 ship) and by the time of "Unimatrix" it had already been augmented by superior tech several times so it wasn't just some normal Fed ship anymore.
Hell, if they had done what Crusher did and use the power of a Star to damage the Tactical Cube (not even destroy it) the reaction would've just been "It's utterly absurd that a Borg ship could be damaged by a solar flare, the Borg should be stronger than a Sun!"
Maybe they should've just used the tiny Probe ship again? Have it be some big life-or-death struggle for the VOY crew to simply survive an encounter with a weak Probe ship, just to show how pathetic they are that they can't beat a Probe whereas Picard and co can destroy an Assimilation Cube, Crusher can destroy the Renegade Borg Cruiser and Sisko can defeat any Dominion vessel he comes across?
Would that be less absurd, making the crew utterly pathetic losers who barely survive run-ins with Probes?