I've played few games with the new expansion, and I haven't found subj to be very useful. I mainly grab them because of a cool model.

Why would they be better then a pack of Reanimates? I feel like 'Minion' alone overweights all of the DK's perks.

They can be very strong, but I've found that they are better with Skeleton Archers for weakening the enemy. And reanimates don't have the same board presence.

Well, first of all, they cannot be attcked by any weak unit. This means that they cannot be countered by any weak unit too, so, if you choose properly your targets, this unit is tough to demolish. Additionally, every retreat means 1 direct damage, so, they have 3 hit results / die when attacking

Plus, as Toenail said, they are fantastic when used in synergy with Skeleton Archers: the archers blight the unit that now is weak and cannot counter in any case the DK's attack.

Honestly, I find them rather impressive

I didn't know about counter, are you sure that counter counts as an attack?

100 % sure. I cannot dig into the rules right now, sorry, possibly Garrett could help me here if you need a rules reference

The closest the rules come is noting that counters "follow the combat sequence for a normal attack" with a few changes (counter must target the original attacker, can't advance or pursue when countering).

But yeah, it's understood that counters are considered attacks.

In terms of Reanimates vs. Death Knights, I like to think of it like this?

Am I fighting in trees? If yes, Reanimates.

Am I fighting legends? If yes, usually Reanimates (legends are never weak, so unless it's a ranged legend or you hava skelly archers then you're losing half the abilities)

Do I have Skeleton Archers? If yes, Death Knights. Lots of synergy.

Am I using necros and graveyards? If yes, generally go for more reanimates.

Am I pushing an objective/hex? Death Knights, getting retreats rolled here is key, DKs will reliably roll 4d giving you more chances at a flag. Reanimates can do this, but the moment they start to take damage, they lose effectiveness.

Honestly, I find the biggest strength of reanimates to be in forests and in a 'minion army'.

The biggest strength of DKs in their reliability and the fact that you order 6 points with one order as with all bigger units. They also synergize wonderfully with a blight heavy army as was pointed out.

Undead are hard for a lot of people I play with at first because you need a more cohesive army to leverage their abilities effectively. With the other factions you can get away with mixing up random units a little more easily (but even then not totally).