If they wish to achieve their goal, they'll use whatever it takes.
Or, more exactly, they use whatever it takes
and nothing more.
When they were attacked via transporter in "BoBW", their adaptation was not to raise shields. It was to erect a special transporter-blocking field, which our heroes could still penetrate with their shuttlecraft.
Also, despite encountering Starfleet hand phasers multiple times, the Borg always start out not wearing a phaserproof shield.
Despite such obvious failures of the doctrine, the Borg seem to persist with it. Never prepare. And always only adapt as much as you really have to.
The Borg do have a desire to remain unseen, or else they wouldn't be the Bigfoot of the Trek universe. But they can meet that goal simply by staying in the deep, distant woods and not making much noise. Invisibility would be overkill, and the Borg never overkill.
Infiltration doesn't appear to be a Borg mission. Confrontation is. If it becomes possible to get a Drone or a Cube into the midst of the enemy, it's always best to reveal it to the enemy, to provoke a confrontation, and to learn from the results. Losses are insignificant - and giving the enemy time to prepare is a strategically important goal, because it's these very preparations that the Borg are after! A successful surprise strike would be a failure by the Borg definition.
Timo Saloniemi