It's a short list of encounters Jack couldn't outright defeat, but Minions are on it.
Funny how the sword didn't affect them, though. I guess a single skyfather being could kill Aku outright though.. It was only the sheer enormity of mass that posed a threat to them, which Aku is only a small piece of..
Yeah, but the short list of people he couldn't defeat basically beat him nearly to death. They're far beyond Jack, who himself proved he was beyond the Daughters, even without his sword. He had it against these guys and was still steamrolled. This is just mean for the Sisters if it was just one of them. But you've got 3 of Jack's worst beatings all in the same place.
And yeah, the Darkness was easily destroyed. It was just so much that it kept growing back. And a tiny piece survived the final battle.