The problem with the Borg is a problem that comes with creating any all-powerful, seemingly unstoppable adversary- eventually, the more times they get used (which happens because they usually get popular, the mystique of that super-opponent), the more they will get degraded, because as villains, it's hard to write the heroes constantly losing or escaping from battle. That can get old and tv writers haven't taken to that concept really. (a Monty Python-esque "Run away! Run away! anytime they see a Borg cube)
If TNG's 2 classic Borg encounters ("I, Borg" was just 1 drone, and the drones retrieiving him, not a standard clash) were in VOY, they might be bashed. Using a deus ex machina to escape being entirely assimilated ("Q Who?") and conveniently stopping the Borg in the Sol solar system by a seemingly simple sleep command ("BOBW").
The more the Borg get encountered, the weaker they would get. The more Borg culture is explored, the more weakpoints would get revealed. Look at Species 8472, same degradation.
"Unity" got such a backlash because people wanted a big Borg clash, not a derelict cube & ex-Borg. They felt cheated. That prompted "Year of Hell" to be shifted from the cliffhanger to in the midst of Season 4 and "Scorpion" to get made as the Season 3 finale. People loved "Scorpion" at the time. "In the Flesh" might've colored people's hindsight on Species 8472.
It takes creative writing to have a weaker protagonist survive multiple encounters with a stronger opponent, which is above most writers' skill. I am reminded of the Batman animated series in the '90s. The Riddler appeared so rarely in there because the writers admitted it was really hard to write a good Riddler story true to the character because he's so intelligent, they felt most schemes they could devise weren't up to the caliber the character's nature demanded.