Let's just run him through the system one thing at a time....
The Duke......he is perhaps the most cost-efficient special character I know with the exception of Vulkan, and like Vulkan he is an army-booster. Unlike the Macho Men (Lysander, Calgar, Logan, Abbadon), The Duke is not going to wipe out armies by his own, let alone survive when he's in combat. I mean, he's no slouch with a rending-ish Venom Blade, but essentially he is weaker than an ordinary Archon statwise just like that of the Fateweaver. But like the fateweaver, what he lacks in statline, he more than make up in his bonus rules.
One of the main rules that make him good is the fact that he allows one to roll two dice for their drug rolls and then you can pick and choose which one you want. With the rolls of drugs being as random as they are, you tend to curse whenever a drug roll result is not to your liking. But with the Duke you can essentially reduce this to an extent and choose the drug result that is closer to the one that you want, if not the result that you want. This is huge for those untis that rely on drugs to a large extent, such as the Wyches, Reavers, Hellions and Beastmasters.
The second reason why he is quite good is because he can essentially give one unit of Kabalite or Trueborn in the army a better poisoned weapon. Wounding on a 3+, every single model in the game would fear the anti-organic firepower that this thing can throw upon its target. Unfortunately this is also where Duke is a little iffy, as the Duke must deploy with an unit of Trueborn or Kabalite if possible but any decent gamer would either make their list to the extent that he will have to put the Duke somewhere, or make their list to an extent that they could attach the Duke to a certain unit that they would want the poisoned rule to come about.
The third rule that makes him quite alright is the fact that he allows any Venom, Raider and Ravager to deepstrike. Now this is quite the bargain as his ability is much better than the retrofire jets upgrades, and this allows for your troops to come out and shoot once they come on the table. It also gives the Duke's army another interesting option to deploy their units.
Before I go on, I am just going to say this first; I never liked playing with special characters. Now that is outside my system. I have played with special characters before but somehow everytime I do so I felt a little restricted as I have to make an army that fits with the strengths of the character, and this dilutes the creativity of making a list. And once people know how good the character is, I feel that I am much more predictable on the tabletop which is something I never liked. Alright, alright, I will admit that I love Mech armies, but each one of my Mech armies work in a different way from one another to the extent that people do not even know that some of these forces could be Mechanized or that Mechanized armies each mechanized army plays differently. But let's move on to my reflection of this particular special character.
For his cost, his abilities made him a very good special character. I find that Sliscus's rules actually does suit my mindset to a large extent, except the fact that he is not as killy as I wanted him to be. But then...the thing about him is that do I honestly need him?
For his poisoned ranged weaponry, I noticed that most if not all of my Kabalites rarely take on the role of killing stuff, and my trueborns are meant to kill tanks and large monsters with their blasters. The killing with poisoned weaponry is the realm of the Venoms for my army and Sliscus' power is only good for the unit he is attached to, not whatever he is riding.
Thirdly, despite the option of deepstriking I noticed that Dark Eldar Vehicles are better off on the table, and their unique shape only serves to make deepstriking as awkward as a Blood Angel Land Raider coming down from the skies in flames of fury. I mean, seriously Why would I want to come down in the middle of my opponent's army for a one-shot suicidal attack before I get ripped to shreds (only Daemons can deepstrike down and not make it a suicide run)?
You are going to suffer, savvy?
And last but not the least, his fluff. His fluff is alright, but nonetheless other than the idea of a sadistic, drug-addicted Captain Jack Sparrow what is he exactly? I mean pirates are cool at times, but the whole Dark Eldar race is a piratical race to begin with, and he doesn't pop up like a sore thumb other than his eccentric behavior.
Some people would think that I am crazy or something, but nonetheless sometimes I have to think about what I need and what I want. Do I honestly need the boost from Sliscus? Do I honestly need to take him just because I can? Do I honestly need to take him just because he's good? Honestly, I do not think so. He can be fun and indeed he could be competitive, but if I can't use his abilities to the fullest, why bother? So far, I can see him getting used once in a while as a count-as for one of the characters of my Kabal, but even then I would not really see him running around too often.