Dark Eldars, while being more honest wit themselves and their motives, do put more value in human's life. They see humans as resource to be tortured for their hedonistic pleasure, and to ward off Slaanesh just a little bit longer by their depravity.
Sure their tech is valuable but as a counterweight we have their inevitable betrayal. Which can lead to a trip into their spiky city, a trip you really don't want to make.
To be Fair, Exodite Eldar are slightly more reasonable. And the Eldar that appear in the RT pnp game are the most reasonable of the lot, being willing to honor their deals, as otherwise no-one would like to play with them or have an adventure that features them.
They have no redeeming values and they'll betray you sooner than later
and... what's the difference with regular spehs elves?
Dark Eldar don't bother pretending they are in the right unlike regular elves, and DE have demonstrated being caapble of researching and advancing their tech much better than the regular Eldar, who have invented... Nothing at all, ever, IIRC.