Biological assimilation isn't the only way the Borg defeat races or adapt to technology. The Borg hadn't assimilated anyone before they adapted to Worf's phasers or the ship's torpedoes in Q Who, for example. And their weapons still pack quite a punch, no matter who you are. You can still get in one or two shots before they adapt as well, no assimilation required to create a defense there.
And before anyone brings up Species 8472, I'd like to point out that they would thoroughly rip apart the Dominion. A single, grazing shot from a mere fighter was able to knock Voyager silly, which is far beyond any sort of firepower we've seen the Dominion unleash, even with phased polaron weapons. Also, we've never seen the Dominion actually blow up a planet as well.
One more point: I'd really like to see Starfleet go up against 15 Cubes. I don't think the results are going to be purdy. See Starfleet go down in flames and we'll get a good idea about how other races will fare.
Actually the Borg had already assimilated Federation technology before Q, Who. You had Seven of Nine's family and ship get taken over and then the attacks in the neutral zone at the beginning of TNG which were attributed to the Borg.
The Dominion haven't been shown to be able to blow up a planet, but they tried to blow up a star.