Succubi aren't bad, but in my eyes they really are more of an addition to a Wych Squad than a true HQ face wrecker. If you are having a low point game, though, a Venom Blade Succubus is wonderfully effective. They're cheap, have plenty of attacks and mow down infantry with ease.
I like them, but I would never take one as my warlord. For a warlord I'd want my nice, survivable Archon with some of the best luck in 40k.
I suppose what people are scared of is the Succubus getting shot down before any real damage can be inflicted. Without the Shadow or Clonefield upgrades available it may seem odd to take one, but I do like them.