I'm not saying there can't be advances against the Borg, but, as BigDaveX points out in the post above mine, they treat it as an ordinary mission, "gonna get assimilated for an afternoon in order to defeat the Borg yet again, BRB!"
Also...the Borg also had 10 years to advance their assimilation techniques...and in First Contact, which happened just before the Voyager episodes (or at least close to it, IDK and IDC about the exact timeline, sorry) assimilation is still treated as this super serious thing.
Except the Borg are also shown, for their big mortal enemy (humans), to keep sending "just one cube"(tm) for every shiny new invasion, when we all know that the Federation barely scrapes by with one cube so two ought to do it. Never mind the number of times the one-liner of "After one or two shots they'll adapt" and six phaser blasts later do they finally do so. Do the Borg not have heuristic stochastic analysis abilities?! "Hey look 3 of 8675309, it's a Federation peep that's just beamed aboard our cube. We saw people like them six years ago, four years ago, three years ago, one year ago, six weeks ago... Are they a threat?" 3 retorts: "No. Keep walking, we're almost to the bathroom." (Seriously, that is dredged up as often as "We are the borg, resistance is futile, you will be assimilated." as if the Collective can't see it's a boatload of humans that identify as "Federation" in front of them. And there's no magic cloak anyone deployed or got discovered.) Or even looking at a phaser pulled out and not realize it's going to be used?!!! The script writers sometimes do get into corners when dealing with "the big bad" and how to keep them either (a) big and bad, and/or (b) interesting. IMHO, VOY kept them interesting far more than STFC had - so for all the nitpicks one can bring up, the overall and lengthy success rate is still important to mention. But most if not all Star Trek incarnations also have the ship using transporters through its own raised shields when plot-convenient as well, which also takes any viewer who's sufficiently invested to know that in every other story told that that's not possible to do...