If the only thing that's good about an enemy is that they're "Mysterious" then that just highlights what a poorly thought out enemy they are.
I didn't say that was the only good thing about them. This was the first enemy that the Federation could not defeat by conventional means. It was one unique android that was able to stop the attacking Borg cube by a fluke. Without Data, it looked like Earth would have been easily assimilated.
True, the Borg have one boring
mission: To assimilate everything. There's no reasoning with them. They are powerful, believe they are the most powerful, and arrogantly explore around the universe looking for civilizations to assimilate.
However, they are relentless. You destroy one cube and more come. We eventually discover they've got special conduits throughout the galaxy, allowing them to easily reach all four quadrants. It looks very much like an intent to dominate the whole galaxy. This makes for a formidable enemy.
In TNG, we see only one Borg cube invade Earth's solar system. Why not more? It could have been that this was an exploration vessel on its own. When it tracked the Enterprise after the first encounter initiated by "Q", it was exploring to see how weak the enemy was. Evaluate and then react as necessary. They were probably arrogant enough to believe they could do it all with one cube. And they were almost
right... only proven wrong, because of Data.
We don't know how thinly spread out the Borg were in TNG, or what other engagements they had going. But clearly the destruction of the first cube wasn't the end of it. We learn later in "I, Borg" that they're still around. And ignoring the Borg faction that was inspired by Hugh, that was it. It wasn't until First Contact that we see the Borg making another incursion to Earth. As to why it wasn't a more aggressive force, well, they may have underestimated things yet again.
Voyager certainly took greater liberties with exploring the Borg. I think it was a great idea to bring them into the story line. I didn't like the idea of the Queen, but I understand the need to make it more "interesting", by having a tangible face. Still, she shouldn't have had such a human personality with emotions clearly on display. And I didn't like how Voyager was able to fend off Borg attacks as they did, given how easily other Federation ships of greater power were destroyed in TNG. True, once 7of9 was on board, Borg technology was used to help improve Voyager's capabilities and make for a greater defense. But this should have been exposed a bit better.
So I don't think Voyager weakened the Borg. Voyager made the Borg story idea much more extensive. And I like how species 8472 was brought in... it helped explain why the Borg weren't flooding the Alpha quadrant more extensively.