Against infantry, it is the range and rate of fire that sets the Dark Eldar apart guns apart. It is very easy for an army to be set up with 30-40 anti-tank weapons, and still maintain a ridicules amount of anti infantry, even going over a 100+ shots a round without counting rifles or pistols.
To match the marine equivalent weapons, you could look at Heavy Bolters or Assault Cannons as your anti-infantry. The rate of fire on that heavy bolter makes it a heavy 3 S5 AP4 at 36". While we all know that the Assault Cannon fires 4 shots at 24" with S6 AP4 rending.
Lets compare.
Splinter Cannon is 36" Poison weapon (4+) AP5 Assault 4 or Heavy 6. Many things like Venoms or Trueborn, or Scourges can carry them in Multiples, Scourges of which can carry 4. Being a Heavy 6 on Venoms, you have two splinter cannons that fire defensively. This means 12" movement and 12 shots per venom 36" away. Even scourges who will get 16 shots when they move, get 24 when sitting still (then they also have carbines).
It doesn't matter if marines are in cover, because they will get their armour save anyways, so this is the gun for them. Pelt them and watch them crumble. Monsterous creatures? lol, who cares we always wound on a 4+. Splinter Cannons have the range and flexibility that no other gun has. In general Splinter Weapons get the guys in cover, while Disintegrators get the guys without cover. Which leads us to Disintegrators.
Disintegrator. Ever wish your heavy bolter was AP2 and mounted on a fast skimmer? Disintegrators are S5 AP2 36" Heavy3. The only come on your heavy support vehicles or Raiders, and are amazing weapons in how they compliment the splinter weaponry in your list. Heavy infantry already has a 2+ save or has Feel No Pain and since lances are single shot,
we have the perfect weapon for you.
I have found ravagers with 3 disintegrators often get ignored often during the firefight, until that is they unleash the first time. I have been fielding two Lance Ravagers with 1 Disintegrator Ravager. It is amazing when you fly your ravager 12" to work your way around cover and then unleash 9 Disintegrator S5 AP2 shots at that unit that just came out of their transport.
Most people do not like the Disintegrator. Most of it due to losing S7 plasma cannon like fire. I personally am still in love with it. It has a role, so you do not need as much of it. However don't worry, because there are only so many places you can take them. In vehicle heavy lists, I generally will use 1 or 2 raiders with them, and like I had said, 1 out of the 3 ravagers.
The Point. These two guns can be found throughout your army, and while most of the dark eldar ranges are set at 18" or 36", we can move and still get those ranges. With so many lances available in your lists, you can match things up perfectly with your anti infantry to truly provide a well combined force, able to take down those vehicles and kill everything inside all within your shooting phase. Splinter Cannons taking down those in cover and forcing armour saves, and Disintegrators for those heavily armoured infantry and those stuck out of cover.