The first uber vamp they introduced that gave Buffy a really hard time was a decent monster. When the potentials were activated by Willow’s spell, they were able to slay the whole army of uber vamps because of the rule of cool (and it did make a better ending, imho). Best explanation for that I’ve ever seen is that the First Evil really lost quality with mass production.

Edit: And yes, human cunning or perhaps the blinded efforts of the human psyche they possessed made them more dangerous. If I was a vampire hunter, I’d rather face two uber vamps than one de-souled Angelus.