Sure, the Zerg have good enough immune systems and adaptability to resist Borg Assimilation like Species 8472 did; but that still doesn't mean that a single Borg cube couldn't solo the entirety of the Zerg. Even the most powerful of all Zerg units, max powered-up Primal Queen of Blades Kerrigan, is only High 6-A AP and Dura, while Borg vessels (of which there are countless thousands) all have At Least 5-A AP and Dura.
In-character, we know from the confrontation with Species 8472 that the Borg don't really know how to defeat their enemy by coming up with good tactics to just kill them, if they can't assimilate them; however, that only really MATTERED because Species 8472 had ships with AP and dura at least on the same tier as the Borg. The Borg were just as happy to simply destroy or drive off Species 8472 as they would normally be to assimilate a species, once they realized that they COULDN'T assimilate Species 8472. It would be the same with the Zerg. For a while, the Borg would stubbornly try to simply assimilate The Zerg in ground confrontations, because obviousy the Zerg are a valuable, genetically superior species with great psionic potential and would be worth assimilating. But, once the Borg eventually realize that they can't assimlate the Zerg under Kerrigan no matter what they do, they will simply send a cube straight to Kerrigan's planet and blow the whole thing into a cloud of vapor.
So the Borg take this. Easily. Not by human perspectives of "easily," but by theirs; they would lose billions or trillions of drones, most likely, during attempts to assimilate the Zerg and engage with Kerrigan using their ground-forces, before finally giving up on that and deciding to just destroy the un-assimilatable threat. But to the Borg Collective, the lives of individual drones don't matter, and losing a few trillion of them would be like the loss of a hand for a character who can regenerate limbs; nothing more than a minor setback.