Let's open the discussion to the first undead unit. Let us start with death Knights:

Executioner: I think this is not the strongest ability. It causes damage instead of retreat. Ok but lot of time i want to cause a retreat and the heroic-ability is quite useless. So I think Executioner is just a little Bonus, but nothing extraordinary. It has a synergy with Unholy Might: If you get the enemy unit weak thanks to executioner, you get a part of the retreat-Bonus back: they lose the counter!

Unholy Mitght: Unholy Might is an interesting ability that may keep you safe your Death Knight. The strong Basic attack of 4 and the several ways of Waiqar to cause "weakness" are making that ability quite powerfull. Being able to attack a blighted full-health unit with 4 dices and not getting any counter is what you want to do with them...

Hidden 3rd ability: FFG, we caught you tricking! What did they do? There is a difference between elite-melee and cavalry units: For the same Price, cavalry gets 1 skill Point more for free! I think the General idea was, that a skill-point on movement may be less important as Attack or Health. So they gave them a free skill on movement with the idea of giving the melee units better or defensiv abilities...

Now: The tricky designers create an cavalry unit with the movement skill (2) of a melee-unit to give them a free attack skill! They are the only non-legendary unit with a basic attack of 4 and movement 2 without any restrictions! So they have a powerfull hidden 3rd ability: add 1 dice to your Basic attack skill of 3!

My opinion:

For the mentioned reason I think that this unit with one relativ weak ability like executioner is a good unit. Look at them as an elite-melee unit with better skills than any other elite melee and one quite weak and one quite good ability.

What is the duty of death Knights? A massive offensiv threat with a good defensiv ability. A unit to knock out your enemies at the same time as you might get a defensiv boost when you cause blightness. So archers and dragon are it's best friends. So they are representing one of the 2 general ways to play Waiqar: Not the Necromancer / Minion-Support tactic but the Dragon-Archers-Knights-blightening way... and here they do a good job!

Having this strong attack and the possibility to avoid a counter, they want to attack and you should try to avoid them getting attacked first. That's not easy to make sure because they will be a priority target of your enemies cavalry and ranged units. So consider this when thinking about your tactic!

Yeah, they are sort of like Ironbound or Obscenes, only their increased movement is always active for free. I suppose you meant non-elite and non-legendary when you mentioned that extra movement and 4 combat dice.

I agree that the Death Knights seem to be in a good place. They have synergy with other Undead units, which is good, but their stats are good enough that if you don't get Blight on your targets from friendly units, it can still be a threat.

I agree that Executioner is a bit weak since it requires at least two particular die faces to cause a single damage. But not being able to counter them is huge!

Yeah, they are sort of like Ironbound or Obscenes, only their increased movement is always active for free. I suppose you meant non-elite and non-legendary when you mentioned that extra movement and 4 combat dice.

I agree that the Death Knights seem to be in a good place. They have synergy with other Undead units, which is good, but their stats are good enough that if you don't get Blight on your targets from friendly units, it can still be a threat.

I agree that Executioner is a bit weak since it requires at least two particular die faces to cause a single damage. But not being able to counter them is huge!

No, I meant non-legendary. There is no other non-legendary unit with attack 4 and movement 2. Obscenes as Ironbounds have a movement of "1+" (or movement 2 with restriction)

The main problem for me is that they compete with formidable Reanimates.

Which:

-cost less

-are better in woods

-are minions

So, unless you are playing a blighting army, which is a heavy limitation, you don't have an incentive to take them. Except an insanely cool model, of course

Yeah, they are sort of like Ironbound or Obscenes, only their increased movement is always active for free. I suppose you meant non-elite and non-legendary when you mentioned that extra movement and 4 combat dice.

I agree that the Death Knights seem to be in a good place. They have synergy with other Undead units, which is good, but their stats are good enough that if you don't get Blight on your targets from friendly units, it can still be a threat.

I agree that Executioner is a bit weak since it requires at least two particular die faces to cause a single damage. But not being able to counter them is huge!

No, I meant non-legendary. There is no other non-legendary unit with attack 4 and movement 2. Obscenes as Ironbounds have a movement of "1+" (or movement 2 with restriction)

Oops. My mistake. My reading comprehension skills have failed me again. At least we were both thinking of the same units.

The main problem for me is that they compete with formidable Reanimates.

Which:

-cost less

-are better in woods

-are minions

So, unless you are playing a blighting army, which is a heavy limitation, you don't have an incentive to take them. Except an insanely cool model, of course

The Reanimates cost 2 less, so you could get 3 of them instead of 2 Death Knights. While that gives you more total health, I don't think it's strictly better. Yes, they do very well in forest hexes because their ability modifies their combat dice, but on the other hand, their combat dice decrease as they take damage. That doesn't happen to the Death Knight. He'll always roll 4 combat dice. Also, the ability to negate a counter is a big deal that should add to his longevity. Sure, if you blight the target before the Death Knight attacks, that's a sure-fire way of ensuring that Unholy Might is active, but that's not the only way. If the Death Knight can deal 2 damage, he's effectively weakened any unit in the game (excluding Legendary units, Berserkers, and the fact that Armor 1 and Immovable 1 decreases his chances dealing 2 damage). He rolls 4 dice, so getting 2 damage doesn't seem unreasonable. Here are the possible rolls that result in 2 damage (ignoring order of dice):

heroic-morale-morale

heroic-morale-cleave

heroic-morale-strike

strike-strike

strike-cleave

cleave-cleave

If the Death Knight can roll these, he's preventing counters which, in my mind, adds to his overall health. If a Reanimates unit dealt 2 damage, they would still be faced with a counter that could cripple them.

On top of this, you only have to focus on moving this one unit around instead of trying to coordinate 3 separate units. On the flip side, 3 Reanimates gets you more possible max damage because you can make 3 attacks in one turn instead of using 2 Death Knights.

Overall, I think the Death Knights provide more consistent damage over the course of the game compared to the Reanimates. I think both are good choices and both become even more potent in their respective archetypes (Necromancer or Blight).

For me, Death Knights vs Reanimates really comes down to army composition as has been mentioned.

In my battlelore group, my opponents usually pick undead, so I speak more from the perspective of an opponent to undead (though I have played as them a few times). One thing I think my opponents do wrong with DKs is that they kinda tend to lead with those. I think that's wrong. Unless they're all you have as frontliners (which is probably a bad idea), then you should probably lead with something else. Unholy Might and Executioner make DKs best as your second wave of units. They can attack without fear of counters assuming you managed to soften the enemy up a little. If nothing else, just hold back the all out attack for a round or 2 and have some skeleton archers volley and try to get some blights or damage out. Just don't charge them at full hp units. DKs have a good chance of doing 2 damage, but 1 damage is also just about as likely depending on the situation. If you mess that up, then you're stuck with a counter and on their next turn they'll probably just finish off the DKs. That's all the rambling I really have to ramble about this unit.