Well Chariots did lose their cannonball hitting power thanks to Steadfast and the popularity of big blocks of cheap units, but they have become much more resilient since they can no longer be destroyed by high Strength hits (though cannons still make short work of them). They can be real dicks as far as I know. However, from what I know Daemons need their Heralds to buff up their units, so for Daemons I've no idea why would it be a good thing to put your Herald on a Chariot. But then again I'm not familiar with Daemons so its just common sense and not experience that makes up my argument.
Tzeentch chariots are great, I used them in a couple of 7th games and I cant see why they wouldnt work in 8th, in fact I think they'll work better. They keep your herald safe- fast enough to get out of the way of enemies and hard enough to take a decent amount of shooting, plus then I gave them the shooting attack (not so good anymore since you cant march and shoot) and the S3 hits for everyone in base contact... it makes it powerful enough to scythe through little units with relative ease (units where steadfast isnt an issue) but can also fill the normal chariot role in 8th- spotting a combat where you will either lose or struggle to win and smash into the flank of it... suddenly giving you something like +5-6 to the combat res and changing the fight to a win for you (with your block unit getting rid of the enemy steadfast).
Slaaneshi chariots can do the same thing, but dont really have the magic to back up being able to just skirt the enemy all game (waiting for the combat they are needed in)... but they do have the bonus of siren, which with the new charge rules are great- you can pull an enemy unit out towords the flank (needing a 6,6 or similar to reach the chariot) meaning that instead of the enemy charging another of your units you get the charge on the enemy, hopefully into their flank (if you do it right)- which is HUGE. Enemy loses their charge bonus, you get the charge, you get a flank and negate ranks... plus all the attacks that the enemy loses. In a standard block on block fight that siren could suddenly have given you +6 res over what would have happend had the enemy charged you, plus the difference in wounds that would have happened.... then if the enemy does survive and reform to face the unit in combat with them the chariot has an easy flank charge to make (and you'll need it if the unit was strong enough to survive a swing of that much).
Of course a standard herald on steed would be fast enough to pull this sort of trick, but is much less survivable and would be almost useless if they charged in without first rejoining a unit.
Khorne chariots are a bit OTT in my opinion. You'll want the heralds in your units giving you the hatred you need (few high strength attacks.. means you need to hit with as many as you possibly can).... but a khorne chariot can target much bigger units all by itself and still expect to win- send it into units of ~15 and then try to get the 6+ kills you need to get rid of steadfast... it would be interesting to see just how big a unit you could attack before it became suicidal (but the randomness of the chariot would make things a bit iffy)... but then if you do fail to break the enemy they are still faced by a herald of khorne.. so unless they have a character or somethign nasty like great weapons (or buffs) you'll still probably win.
Basically I love daemon chariots and would find uses for any in an army I was running... but they arent as simple as they use to be... simply smashing them straight forwards into the closest enemy unit isnt going to even vaguely work anymore, but give it a bit of thought and they're great.
I do like the Tzeentch Caster on a Chariot to keep him away from miscasting your units to death, makes him hardier and faster to get to trouble spots. You can March and Cast as well so thats a great boon.
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