A point we've been trying to hammer home lately is that Grey Knights are Marines with extra bling. This is part of their army balance as in a one on one fight, they are better than most of their Marine counterparts due to their extra equipment. This makes them more expensive but with the same statline which equates to them dying at the same mathematical rate as the other variants. This is a limitation of the army as you generally end up with less overall units than other Marine armies and often less bodies or army flexibility because of this. This is a key reason why Grey Knights aren't a combat army and has been discussed extensively, use combat as back-up to their much more impressive shooting.
All that being said however there is an assumption here. Grey Knights die the same as Marines according to mathhammer. If I have 20 attacks at Strengh and WS X, I'm going to inflict Z wounds on T4/3+ opponents. What this doesn't take into account is that Grey Knights are often doing more initial damage than their Marine counterparts. They are more expensive for a reason. Sethis explained this very well so I've copy-pasted his reply in a recent post here:
"A niggling point that bothered me when reading this article: You keep repeating that GKs die like Marines, which is true on a "stats" level i.e. something that hits marines on 3s and kills marines on 3s will do the same to GKs BUT you haven't mentioned at all is the fact that over several rounds of combat (or even during the first one if the GKs are higher I than their opponent) the GKs will take many less casualties than their equivalent points of Marines.
A totally Vanilla Tactical squad fighting 20 Guardsmen, for example. Tacs strike first, and lets assume they have the charge. So 10 pistol shots which kills about 2 Guardsmen. 18 left. 20 CC attacks kill about 5 Guard, leaving 13 men to strike back.
Compare vanilla GKs who do the same. 20 SB shots kills about 5 Guard, followed by 20 CC attacks that kill about 12 Guardsmen with Hammerhand. Even without Hammerhand they kill 8, leaving 7 to strike back. The difference isn't how much they win combat by, but how many fewer attacks they take in return, and therefore less casualties. A full Halberd squad exemplifies this even more, because they also get to strike before a lot of dedicated CC units such as FC Marines like Zerkers/BA/Eldar, instead of just before Guard/Tau etc. A pure Halberd squad will almost wipe out a Scorpion squad before they can strike, whereas normal marines have to eat 30 attacks before doing so.
They even get an advantage when striking after their opponents, because the additional casualties caused by ignoring armour (a third more kills vs 5+, half again more kills vs 4+, and triple the kills vs 3+ etc) will affect the future rounds of combat.
Less attackers striking = less wounds taken = living longer. You are correct in that any given opponent will need the same dice rolls to kill a GK as a Marine when he strikes, and in that respect they are equally fragile, but you have to take into account that there will be less models to attack the GKs than there would be to attack the Marines in the first place."
This is absolutely true and an assumption we have made assuming you would understand this but it is always good to point these things out. In terms of one on one fights, Grey Knights are often going to have the advantage against other generalist units. They are the ultimate generalists in a sense being able to shoot well, take damage well and do combat pretty decently and although they are expensive at what they do, will pulp most other generalist units into the ground. The problem is when you face specialist units one on one. Specifically, combat specialist units. They are now going to beat you to a pulp and don't really care that you're at a minimum 4 points more expensive than your regular Tactical Marine. They may take a bit more damage from Hammerhand/Force Weapons but they are still going to beat you. This loss of a unit is going to be felt more in a Grey Knight army because of the way they are set up and this is often what we are trying to highlight when it is said that Grey Knights die like Marines.
I hope that has cleared this up some and thanks to Sethis for pointing it out and writing an eloquent response on it.
