The difference with Dark Eldar is, there is no variety. You can disparage anything and everything as "spam", but a Draigo/Paladin list plays distinctively different to a Henchmen list, which plays distinctively different to a Stormraven list, which plays distinctively different to a Purifier-list, etc.. . There is room for marketedly different play-styles that, yes, can be taken to extremes or, yes, can be combined in hybrid lists.
DE fails in that because the basic concept never changes. Whatever you load into the Venoms/Raiders, it relies on overwhelming the opponent with target-saturation and speed as/after you demech them with (mostly) Ravagers (from possibly the worst internal balance for HS in any 40K Codex save Long Fangs).
It's a one-trick pony whichever way you spin it. Variety in DE is "you can have any colour as long as its black". Thus.. spam.
And in relation to the latter, it should be noted that DE are not described as Horde-army. They are described as manipulative and of using advanced mobility and dark technology to gain supremacy. That is utterly lost as they essentially outnumber (in units, if not in models) pretty much any other army out there and simply win by overwhelming. The list in DE utterly fails to capture the nature of the army in fluff and background. The only other Codex that fails in similar ways is Space Wolves, where Long Fangs and JoTWW turned "into-the-teeth-of-the-enemy-heroes" (fluff) into a "sit-back, relax, character-sniping & gunlines"-army (gameplay)
Thus, Dark Eldar and Space Wolves are by and large at the bottom of 5th Edition books. Neither comes close to the quality of Grey Knights (or most other 5th Edition Codexes).
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Also, the Grey Knight fluff is top-notch. I am not sure what your problem is, but the book took one of the most dull, thematically convoluted and pretentious older books and revived them with a strong, unique theme, great grimdark stories, iconic characters and fantastic tie-ins to both the popular HH-novels and other Codexes (incl. the latter-coming Necrons).
In all of these it is far superiour to, among others, the DE Codex with languishes without strong interconnection with the other fluff, suffers from n extremely boring, list-less and expositonary writing style (e.g. "Vect is the most intelligent DE" , "DE are the most depraved race".. screw that.. don't tell me, SHOW ME you moron of a writer) and some of the most hare-brained naming ever seen (Decapitator-wielding Decapitator decapitating people?). The stories included grate with 40K sensibilities by catering more to a loony-toon-bugs-bunny aesthetic than a 40K one (e.g. "black-holes in a box") and seem to seep too much with the writer's own unresolved childhood issue (poor Vect became "evil" because he wasn't taken serious as a child? Cry me a river).