Not to mention that most of what we see of the Abyss is more on the "nightmarish horror" side; Prince of 100,000 Leaves anyone?

In my view, this isn't because the Abyss is evil per se, but because it is random. Think about it like a simplified view of entropy; there are a small number of "ordered" bits of the Abyss which humans can thrive and live comfortably, and a nigh-infinite number of "disordered" ones which are horrific in one way or another. Any time you introduce a bit of Abyss into the Fallen World, especially in an uncontrolled way like Paradox, you are almost certainly making it a little less habitable for humans.