Borg Drones can adapt, but they can't constantly be adapted to everything. They're the heavy infantry of Star Trek, marching slowly, taking hits, slowly grinding their enemies down.
The early Roman Republic, emerging from the previous kingdom, adopted the Greek Phalanx from its earlier cruder formations and formed a strong cohesive heavy infantry. Their neighbors on the Italian peninsula did not have an answer for it, nor the kind of cohesive military to manage their own phalanxi, so they were routinely beaten. But the Phalanx had problems when it encountered other phalanx, or when it met mountainous topography. Rome did not pull a Sparta and just stick with it, but adapted, into the maniple, and later cohort legion systems. These were more adaptable. Each time Rome was beaten it learned from the lesson and adapted. By 53 BC the Romans were knocking on the doors of the East. Their first major engagement with the Parthian Empire at Carrahae was a catastrophic loss. The Romans had had hundreds of years in their territories of heavy infantry ruling the battlefield. The Persians had been beaten by the phalanx centuries earlier and had learned how to get around it. Battles would go back and forth for centuries, but Rome never expanded deep into Persian territory.
sorry I digress. The TR-116 was interesting thinking by Starfleet. It wouldn't work repeatedly, but as a sniper weapon (when it doesn't really matter who you shoot, the benefit of drones, no officers), it does allow you to get a few hits in prior to adaptation. An attack on the Borg would seem to need to be using multiple weapon types, in hit and run tactics, outflanking, causing harm, retreating before taking heavy casualties but returning to hit before the Borg could regenerate or regroup.
