The Borg were very badly damaged by Fed tech in their first appearance. They needed to drain the Enterprise database for information and slice a chunk of the ship out to analyze it before they could adapt to anything. AND they had to repair the damage to their ship first.
And if the guys they're fighting have weaponry on par with or superior to Borg tech, then it's debatable IF they Borg can adapt at all.
The Borg were never meant to be as overpowered as the fandom made them out to be.
Personally, I'd have introduced the 8472 aliens in TNG. I'd have introduced them in "Q Who?" itself: Q teleports the Enterprise into the middle of a battle between the 8472 and the Borg, with the Borg losing. The Enterprise runs away and one of the Cubes chases them.
It would make it clear that the Borg's adaptive powers works well on the Feds because they're clearly inferior to the Borg. But if the Borg are fighting with someone equal to them, no dice.