If you play skirmishes in Soulstorm I highly recommend installing the unofficial patch, and also the Dawn of Skirmish A.I. mod which dramatically makes the A.I. more challenging (as a side note I myself was a beta tester for that mod). First, you install the official patches in order, 1.10, and then 1.20. After that you install the Bugfix Mod (latest version from what I know right now is 1.41) specifically for the Soulstorm expansion (if they still have separate versions of that mod that is). Then finally you install Dawn of Skirmish 3.20 and you're set. I don't have the direct links for the mods right now but I will provide them if you can't find them.
As for the Dark Eldar in Soulstorm specifically I think they're alright at least when it comes to skirmishes. If we're talking about on-line skirmishes against other players then I wouldn't pick the Dark Eldar, I'd rather go with the Eldar (Craftworld), Space Marines, IG or Chaos. I don't really like the Sisters of Battle, Necrons or Tau. As a side note, set the difficulty of the A.I. from the DoS mod to the highest and play against 3 or 4 Necrons (with 3 or 4 allies that is) and give me some news about that, especially on maps with lots of Requisition points and Relics. I can easily have hours-long skirmishes under similar settings, quite a blast.