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To back up a step, I believe that strong codicies have internal synergies that can be leveraged in army composition and play. I'm ignoring the super-friends aspect of 6th and 7th editions here, as I feel that any codex should be able to compete on its own merits, but I digress.
Take the 7th ed Necron book. There are numerous examples of explicit synergy, where a model or unit directly empowers one or more friendly units by fiat. The ability of a Triach Stalker to boost the BS of nearby Necrons, or a Cryptek to improve a unit's save, or the Overlord of a Reclamation Legion to enhance a unit's reanimation protocols are all examples of explicit synergy. As the benefits from this kind of overt bonus are clear, we can expect that to be reflected in the points cost of the unit providing the bonus, at least to some extent.
However, there is also a layer of implicit synergy that underlies strong performers. More subtle than a special rule that makes things better, this is where units complement each other to improve strengths and mitigate weaknesses, or where a unit's different options allow it to build on its own strengths, especially in combination. To continue with Necron examples, a Necron Warrior's critical weakness to melee combat (or more precisely, from being swept in a losing melee) can be mitigated by the inclusion of fearless melee troops like Praetorians or Wraiths, or even an overlord wit ha lucky warlord trait roll. In return, the melee troops benefit from the presence of a blob of warriors suppressing their preferred targets or forcibly debarking them with gauss fire.
Harnessing the implicit synergy available to your faction is the basis of good list design. More strong lists can be made from a codex that has good implicit synergy than from a book with poor implicit synergy. Very powerful codicies tend to have solid amounts of both implicit and explicit synergy. Hence, the era of the 'Decurion' or meta-formation codex has heralded books that can make their predeccessors feel like they have brought a butter knife to a gunfight.
Which brings me to the Dark Eldar, and the dissynergies that leave them resembling a butter knife made of glass.
The Dark Eldar are not completely without synergy; indeed, if they were they would be completely unplayable. A notable if expensive highlight is the Dark Artisan formation, which allows a haemonculus to deep strike a pair of monstrous creatures. Nor are they entirely without good units, as Grotesques will attest. But the frequent instances of dissynergy act as a brake on large portions of the book.
The Cronos is great in a Dark Artisan. It's boost to feel no pain rolls is explicitly synergistic. Yet to get that boost it has to forego the opportunity to take a long(er) ranged weapon, leaving it with only a template. So, while it would be an excellent melee support unit it is too slow to get there, and if used in support of ranged troops can't actually contribute to shooting itself. The constraints from its shortcomings generally prevent it from being in the right position to use its boosts, taking a unit presumably intended to provide explicit synergy and instead making it a locus of implicit dissynergy.
Archons certainly lost their teeth in 7ed. Their inability to access an AP2 melee weapon has also had knock on effects, meaning that they are no longer suited to running with Incubi, as they will merely bounce off the harder targets that Incubi can engage. As they are no longer able to provide the Incubi with grenades, the relationship between an Archon and his bodyguards has never been more distant. At least he can embrace a new role as the purveyor of webway portals to units interested in shining darklight on enemies.
The Succubus has taken over from the Archon as the melee HQ character, thanks to the ability to actually equip an AP2 weapon, at S4 no less. Yes, the cream of one on one killing in Commorragh are actually able to output attacks as potent as Sgt Joe Blow from Cadia and his funky axe, but that's another rant. This AP2 makes her an ok choice to run with another unit that can dish it out in combat, like the aforementioned Incubi, Grotesques or even her own people, Wyches. Except that Grotesques, with their S5 poison, will happily charge monstrous creatures and other targets that she can't hurt, and even wyches with an agoniser-toting hekatrix will be happy to charge T6 beasties that leave the poor Succy looking decidedly... well, sucky. So it seems that she maybe wants to run with Incubi after all. Unless you take the agoniser as well, spending 45 points to make her a half decent all-rounder. Constraints, limits, and dissynergy.
Some of the worst offenders are in the wargear section. The soul trap? +1 S for every wound inflicted in a challenge isn't too bad, it would synergise very nicely with a Succubus with an Archite glaive, allowing her to become very handy indeed after a combat or too. Which is why she can't take it. Only the Archon, who is no longer all that great in combat, may take this combat oriented item, so that he may increase his strength while hitting with his poisoned agoniser, which does not use his strength. A textbook example of an opportunity for synergy lost, and a dissynergy gained. Let's not even talk about the Djinn blade.
The soul trap is in fact a template for a large part of the dissynergy evident in the book; an item which has potential squandered by not being available to the units who would actually be able to take advantage of it. I've complained repeatedly about the siloing of heavy and special weapons options, which falls under this category. The bizarre availability of venom blades to only solarites and acothysts, to neither of whom it is particularly useful, is another case in point. A hekatrix or even the odd sybarite would love to get their hands on a venom blade, like they used to be able to...
The voidraven has the highest strength ranged weapons that the army has access to, making it the perfect candidate for hunting vehicles, and is able to upgrade to missiles that hurt infantry and bounce off tanks. It is the best candidate for attacking enemy air assets that we have, and none of it's upgrade options can even be used against planes.
The razorwing can certainly be geared to take out infantry. In fact it has three missile options to do more or less the same thing, in putting a bunch of low AP wounds on infantry with a large blast marker. Why it has three different ways to achieve the same result when what it needs is one way to kill infantry to go with its disintegrator option and a second way to kill planes and tanks to go with its dark lances and status as a fighter I do not know, but I do know it doesn't help.
The airplanes in general are broadly dissynergistic with the rest of the Dark Eldar army, by offering more of what the ground forces can already do, while not advancing their capacity to do what is needed, ie destroy planes.
Wyches and combat. Leaving aside the sad fact that wyches aren't all that great in the first place, the 7ed ruling that overwatch may not be taken by pinned units seemed at first an answer to the conundrum of how to get the fragile Wyches into melee at all. Then, in the new Codex, all pinning weapons were removed. Why Mandrakes thought it would be a better idea to set enemies' toes on fire than put their heads down I can't say, but it was another perfect opportunity to create implicit synergy that was missed.
Indeed, we are stuck with a codex that contains two melee units for every shooting unit, but that has no means by which those melee units can help each other. Ideally, the few units that have grenades would be usefully employed softening the enemy before the heavy hitters arrive into a combat already started to finish it off. Except that the units who do have grenades need the heavy hitters to charge first and soak overwatch.
The Power from Pain table provides most of its benefits to melee units. So, naturally, the formation that provides a conditional advancement on the table dictates that you predominantly take shooting units.
I could go on. But it's already painfully clear. The dissynergies resulting from the (often arbitrary) constraints on the Dark Eldar codex make an already challenging army even more so. As appropriate as it may be for a race of self absorbed bastards to not play well with each other, it doesn't make for a parsimonius game experience. We don't need a story with a moral, we need a balanced faction.
Dark Eldar might never be on the power level of Necrons, or Craftworld Eldar. To be honest I hope we never become an easy win button. But I would like to see us balanced, with fewer bad units and choices. Once the dissynergies are adressed we will have a chance at utilising implicit synergy to create lists that at least feel like the glass cannons they should.