The Daemons codex is actually fantastic. Aside from the Troops and HQ slots, you almost can't go wrong with the choices in this book. While some are clearly stronger than others, almost everything has its competitive place in games. With that in mind, it's important to note that every .2 points here counts - scores should end up tight. In place of standard loadouts, I'm including simple notes on the unit. If there's anything that really needs to be said about how to run them, I'll mention it.
Flamers of Tzeentch
S4AP4 flamer on every model
Not awful fast, meaning they should deepstrike
Doesn't really take advantage of their powers, don't have a good way to get close to their enemy.Don't really hit very hard at all.
They'll perform against hordes and swarms, 9 S4AP4 flamers will usually annihilate any squad you put in front of them. The flipside is that more than half of the armies you'll play don't really feel AP4 too badly, and if they do they'll be able to beat it up fairly easily.
One note is that the pyrocaster is BS5, which means that he might make a fairly resilient quadgunner.
Price 4/10 They don't perform well enough to justify their price.
Survivability 6/10 They're dead quickly if they're a threat, they don't matter if they aren't.
Damage Output 5/10 50:50, they perform.
Rules 5/10 Nothing of Note
God's boon bonus 5/10 I guess they make use of their re-rolls ones on saves?
Overall: 5/10 They're not awful, but they don't perform It's a coin flip as to whether or not they're worthwhile, and unfortunately usually that will not work in their favour.
Phenominal on the charge with the right set up.
S6AP3 OTC, 4 attacks each + hammer of Wrath
Even with AP3, they can still drown most 2+ saves in enough attacks that they have to start taking wounds.
Banner of Blood is fantastic in ensuring that charges go off.
T4 really hurts against some armies.
They really, really, really REALLY need a 3+ armour save. Otherwise, losing grimoire on them just hurts too much.
I could write a thesis on what makes bloodcrushers viable in this edition, but I don't think I have your attention for that length of time. I'll keep it simple, to be brought up at a later date (or in the comments when you call me a retard/something else grossly inappropriate) Bloodcrushers are missing something to make them viable. If they were Toughness 5, OR had a 3+ save, OR had AP2, OR had a points reduction (to about 35 points), they would become extremely viable. As it stands now, they're still fantastic, they're just outshone by the other fantastic units in the army, which don't require such complex handling.
Price 5/10 They're just overpriced, but it's not that they're price isn't correct, they just lack something to make their price reasonable.
Survivability 6/10 T4 is shucks if they're bringing a lot of S8, but that just means you have a first turn priority to protect them from it. They're fucked against dark eldar packing too much S8, but they can usually stand up to armies that pack anything but it. It takes 9 wounds to bring one down with a 3++, which is pretty useful for soaking wounds.
Damage Output 10/10 Simply Fantastic, truly. 6 of them puts out 6 S5AP- attacks, 24 S6AP3 attacks on the charge. IF you throw in karanak, and a pair of heralds along with a bloodhunter, you're looking at 8S5AP- 20 S6AP3 attacks, 5 S6AP- attacks, 15 S6AP2 attacks with instant death. Against armies that aren't packing much Sv2+, you can get a massive amount of other benefits on your heralds which can be fantastic for pushing the game in your favour.
Rules 7/10 They make good use of the banner of blood. They also make excellent use of rage or hatred, along with furious charge and the cavalry rules. It's a damn shame they can't have hatred and rage.
God's Boons Bonus 6/10 They make good use of their Furious charge. Why can't khorne give us something else? Shame khorne only gives the one thing.
Overall 7/10 I'm biased and I'm not going to pretend I'm not. I simply love these units, I just wish they were better.
Beasts of Nurgle
Fantastic Damage output
Wonderful rules for pissing off your opponents
Very tough even without grimoire
Access to Locusi is annoying, if possible.
Weak Vs. Vehicles.
Beasts of nurgle are actually fantastic. They're brilliant as a concept, as fluff, as units and as damage soakers. They even do damage dealing relatively well. Let's leave it at this: BONs are wonderful and every one is unique and puppylike and if you don't like them, you're not playing the right army.
Price 8/10 Pricey, but definitely worth it.
Survivability 10/10 Technically beaten out by the great unclean one, but 45 T5 wounds with a 5++, Shrouded, IWND and possibly FNP are not going to be removed from the table. Just watch out for S10.
Damage Output 8/10 Very respectable.
Rules 7/10 Slow and purposeful doesn't hurt them too much, and Attentionseeker can absolutely swing the game in your favour. Slime Trail also means that beasts of nurgle are almost impossible to remove from the table by any means.
God's Boons Bonus 8/10 They like Shrouded, but slow and purposeful hurts a bit.
Overall 8.2/10 Easily the best in the elites slot. They're a awesome frustration unit. Even units of 3 of them will perform.
I6 and Soporific Musk means they almost always go first in close combat.
4 S4 Rending attacks at WS4 each
can usually be left to their own devices, they don't specifically need a Herald.
Siren Song can be annoying
More wounds per point than seekers (marginally)
Take more hits that aren't S8 than seekers.
Less points per attack than seekers (by a landslide)
Fiends don't compare to seekers, which makes it counter-intuitive to compare the two. While seekers hit harder against more things, it can be argued that fiends are more reliable against their targets by always hitting first, but this is where their strengths end. They need the charge set up better and need to be protected on their way to their assault. They make for a half-decent fire-soak if the game doesn't suit them.
Price 6/10 Pricey for what they're intended for, if not unreasonable.
Survivability 7/10 Their only real strength. They hit fast and don't die that easily before they get there.
Damage Output 7/10 Reasonable.
Rules 7/10 Pretty good, really. They hit hard and fast because of them.
God's Boons Bonus 7/10 They make use of that rending very well.
Overall 6.8/10 They aren't definitive, but they aren't awful.
Beasts of nurgle, 3.2 from the worst contender and 1.2 from their closest contender and 1.45 above the average for this section. These guys are fantastic, and should be viewed as such. They're extremely hard to rub out without the right tools, and take a bunch of hits before they die. They need very little handling and that makes them even more reliable. While they come down a little against MC and really feel daemonic instability, Beasts of nurgle can hold out for the whole game and while you shouldn't always expect them to win combat, they will always perform, every game.