The previous generation of the Daemon codex featured an Elites section packed with undercosted, high performing units. There was not really any comparison between the Elites slots with any others - it was obviously the best. Although the names of the units remained unchanged - all four elites selections now play completely differently from their previous incarnations - I think it is actually confusing that they have the same names as before.
Overall, the Elites slot is well balanced - all four options have their uses and it does not make sense to simply replicate one of the selections 3 times and call it an army - unlike the previous codex or updated codex. I have included some hastily snapped photos of my progress on each unit along with an analysis of it on the field.
What a huge change to these guys. They retained their killyness - albeit nerfed slightly when power weapons became AP 3 - but lost survivability for speed. Based on the Chaos Space Marine update this was not totally unexpected - but it really changes the way to use a Crusher unit. Previously I used crushers as an Anvil - a slow killy survivable unit to grind an opponent against. Now they are more a hammer with 3 wounds, T4, 5++ WS 5 and 3 AP 3 attacks at S5 (or S6 on the charge). They are still going to decimate enemies and really need to be evaluated in the context of the buffs they can gain from the rest of the army.
Forewarning gives them a 4++, Endurance a 5++ and It Will Not Die! (which is awesome on multiwound models), a Grimoire makes their save 3++ (or sometimes 2++) and invisibility nets a 2+ cover save. Without these buffs Crushers are overpriced - especially given their vulnerability to instant death from S8 or higher. With these options they are a huge, fast threat that can become a true nightmare for opponents.
They are incredible in the right list - in the wrong list they will die by their 2nd combat from shooting/overwatch/return strikes. I think the way to run them is in units of 4-6 depending on who runs with them as an HQ. Karanak can tank ID shots, lets them scout and provides a Rage Locus, a Khorne Herald provides a T5 body to do the same and an AP 2 weapon (plus a locus - although he gets pricey for all that), and Skulltake can also tank ID hits as the only Eternal Warrior in the codex and provides a nice body for challenges.
Either way, the Daemon list is about maximizing Synergy so I think a Telepathy Psyker, Divination Psyker and Grimoire is a must have. A biomancy psyker is nice for Endurance - but the odds of grabbing that particular buff is not worth dedicating a buffer to it. Skull Cannon or Fiends also provide nice synergy with Crushers lack of grenades - but with all the survivability buffs they should be getting, are also a nice 'like to have.'
Flamers of Tzeentch
Of all the Elites, the role of Flamers is the least changed in their current incarnation. They do not provide much against vehicles - although with S4 I have finished glancing out tanks with them. Otherwise, they still tend to murder troop units - even a unit of Marines does not like to take 25-30 S4 flamer hits - but require more finesse to play. Against T3 with a 4+ or worse Flamers actually perform better than they previously did - in spite of being slightly more susceptible to Instant Death. Given their cost though - a unit of 6 is a nice addition to any list to round it out.
I would not bring more than one unit of Flamers and I would stick to that magic number of 6 - more than that tend to get in their own way. The same concept applied for survivability to Flamers as Crusher - especially if they can be pushed to a 2++ save, they become annoyingly durable. Even at 3++ they are also obnoxious to clear away.
There are not really any clear choice characters to join to Flamers - although a Tzeentch Herald on Disc provides a nice Deep Strike base for them and the extra range on its shooting lets them reach out past their flame template (see the 40k BRB FAQ on this one), just remember to take at least Flickering Fire on the Herald to make it work.
Beasts of Nurgle
I unfortunately - or maybe fortunately given their old rules - do not own any Beasts of Nurgle, although I would recommend the Forgeworld Toads as models for them. Beasts of Nurgle really have three purposes: 1) Generate cover saves, 2) Tie up gunlines, 3) Eat overwatch. They have durability from shooting and It Will Not Die is great on a 4 wound model - especially units of 4 wound models. Slime Tail helps reduce the impact of Furious Charge - but otherwise the natural Nurgle Defensive Grenades mitigates most the damage.
Since Beasts really lack the ability to put down damage on an enemy (d6 + 1 poisoned attacks at S4, WS 3) and may only take a few wounds back due to T5. Unfortunately, Daemonic Instability makes a mockery of T5 or other survival gimmicks. As such, beasts work best as a small unit (I would take no more than 3) that are fast - in spite of being unable to run - and can sit in terrain to tank damage, or to tie up adversaries. Remember, defensive grenades add another +1 to cover saves for enemies within 8" so Beasts will be at a 2+ in area terrain before a charge. The only real problem with them is that Plague Drones bring almost the same skill set for less points but with more speed.
Fiends of Slaanesh
Similar to Blood Crushers, the Fiends can no longer operate as a solo act to take down enemy units. Where crushers are a 'smash face' unit, Fiends are really a support unit with acceptable, but underwhelming melee abilities. With 3 wounds and T 4 they really need the same kind of help as Crushers for defense. On the charge the reduce enemy initiative by 5, which usually results in Daemon units striking first (or if going through terrain) simultaneously. Offensively they dish at S4 with 3 attacks on WS 4 - nothing to write home about - but a unit of 6 - by the way I would run them in units of 6 to facilitate multi charges and ensure some make it to combat - can expect 2 Rends and 4 wounds on T4 when charging. At typical point levels, I would run 1 unit of Fiends - but only if I decided not to run other support choices like a Skull Cannon.
The side bonus of -1 to psychic checks for units within 12" of a Fiend provides an occasional boost when facing psykers, but as a Witch Hunter player in the past - a variant of Hexxagramic Wards are nothing to write home about. If you have not heard of Hexxagramic Wards, then exactly.
Overall, the Elites slot is fairly well balanced internally and against the rest of the codex. They are not an auto-max out slot - I think the most replaceable units are Fiends and Beasts of Nurgle - where Crushers and Flamers both bring something unique to the army that is not easily replicated in other selections. As someone who tends to run a wide range of units and force organization slots, I like the diversity it provides across an army list.