The Dark Eldar book is in my opinion, the ultimate killer of large and/or elitist units but when it comes to the small, generalistic things, they fare much worse. This is true in the sense that when you see how the weaponry of the book works, they are meant to destroy things that are significantly larger than themselves. Dark Lances, Blasters, Splinter Rifles....they are truly amazing against the larger things that we would encounter in the 40k gaming scene, such as Trygons and Land Raiders, but when faced with the prospect of a simple humble guardsmen or a Rhino, and you would see that for some odd reason the weapons actually struggles with them.
With our superior speed, firepower and assault capabilities, you would expect something in the idea that anything that would kill something big would be good against something small. Wrong on so many levels, in my honest opinion as sometimes overkilling is a bad waste of resource and we could do something that is more economically feasible. This is akin to someone using a sledgehammer to crack open a nut than someone using a nutcracker to crack the nut. The nutcracker is a finer tool, and with a finer economy of resource would crack the nut at just the right strength to take out the juicy morsel. Now compare that with a sledgehammer; a sledgehammer smashing on a card is a waste of overwhelming crushing ability and due to its rather uneconomical use of power, the nut and its juicy inside are destroyed altogether.
With that in mind, here are some of the lists that I would enjoy using my Dark Eldar against, and what I don't want to face with my Dark Eldar.
What I love to face;
Double-Lash/plague marine/ Oblit spam
Land Raider Spam
I would do assessment of what I do with units like those featured above in a poetic form of sorts;
With lances I break the backs of Nob Bikers and Land Raiders,
With splinter weaponry, I mow down Trygons and Thunderwolves
With speed, I make a mockery of Swarmlord and Spyders,
With certainty, I rip the heart of those that has the courage of becoming heroes.
What I would hate to face;
Imperial Guard (especially Mech IG)
Why was this the only one being singled out for the purpose of my discomfort in facing this? I would honestly put this in the word of Vakthahi from Warseer when it comes to weird situations against armies such as IG;
I honestly have a hard time thinking of a worse army for DE to have to face than mech IG. The popular Soul-trap/huskblade combo on your HQ? pointless (most IG armies have no IC's and there are no MC's in the army). Expensive incubi unit? Overkill against infantry and worthless against tanks. Wyches/Wracks? You don't need anything that CC specialized to smash guardsmen in CC. Heat Lances? Worse against AV12 than a normal meltagun in terms of average armor penetration. 4+ Poisoned weapons on everything? Sorta weak against T3 5+sv infantry (awesome when one can rapid fire a wraithlord to death with basic guns, not so awesome when they aren't that amazing against basic IG infantry). Power through Pain? Great, you have to shoot up a bunch of tanks, *then* kill a bunch of infantry to get a pain token, which gives DE units FNP, negated by the massed Autocannon/Multilaser fire and anything heavier that IG tend to bring in droves (and the second and third pain tokens are of far more dubious value). The basic Chimera is a nightmare for DE. Lots of Dark Lances? Great, they hit on a 3, glance AV12 on a 4 and pen on a 5 and kill on a 5+ penn. Multilasers hit only on a 4, but glance/pen on the same value back, but kill on a 4+ pen/6+ glance, with *3* times as many shots, and Heavy Bolters can kill any vehicle in the army as well. Blasters? They do to IG tanks and infantry what *grenade launchers* do back to DE vehicles and infantry (aside from the Lance being incredibly useful against Leman Russ tanks, definite point there) with twice the range and a Blast mode.
DE aren't a bad army at all, rather they look to be very good against most opponents with the exception of IG, and most especially mech IG. If anything, I'd expect to seem them push many IG players *to* mech instead of *away* from mech. DE are an army that is wonderful at destroying large units, very tough units, IC's/MC's, and expensive and highly capable units. They fall flat very hard when trying to deal with lots of weeny stuff, as most of their stuff is grossly overkill and results in being rather wasted points.
Honestly, I believe that any GeQ armies would give our Dark Eldar a hard time, as I do not see how Dark Eldar's rather specialist gear and units will kill generalists like IGs, especially when the right amount of Chimera and guardsmen are spammed together in harmony. Gaunt spam I can handle as I need to kill the Synapses and I am fine, but when one has a decentralized frame like that of an IG army, it is a bitch indeed.
Ork spam is something I could handle to an extend, for dark eldar's poisoned weapons still work to a degree with them, but IG only makes it worse for the poisoned as they are somehow degraded to a lasgun for wounding and that sucked balls for anti-infantry purposes.
So with that I know what I love to fight against, and what I hate to fight against using my DE. Farewell.