If Picard had gone ahead and sent Hugh back with the virus to infect the Borg, would it have resulted in anything? Certainly Starfleet thought so. The Borg are expansive and ancient. They have survived for centuries. It seems that something like that would have been a hit, but something the Borg could overcome beyond the scope of the Federation understanding of them. Then again, the Changeling virus was devastating to that species, and it had also survived, expanded, and overcome for eons.