These two units serve very different purposes in a Salamander army and as such will be more or less effective depending on your army composition and direction.
Okay, so kicking off we have Pyroclasts. These guys are artificer armoured pseudo terminators. Coming with artificer armour and a 5++ invulnerable against melta, plasma, flamer and Volkite weapons these guys are pretty well protected. (I don't see the last two being overly helpful but it's nice to have, I guess...)
With basic marine stats, these guys don't have an awful amount going for them besides their apparent durability. Coming with flamer weapons, Pyroclasts come equipped with basic flamers (Str5 because of Promethean Gift), but with a twist, they may also make a 6" melta attack at Str6.
In my honest opinion, this attack is nearly worthless. At a 6" max range you're only going to get the extra D6 within 3", and with only Str6 the average score is 13, meaning you're only going to be penetrating the side and rear armour of light-medium vehicles. Given the short range of these weapons, it's frankly unlikely that you'll get in range of your target before you get shot up.
You can take a land raider to mitigate this range issue but that land raider is probably better spent on a unit that will be more effective.
While they can be effective anti-horde, Heavy Support Squads do this for 10 points less and have Str6 so they'll be causing instant death. Yes, they don't have 2+ armour but really I don't see that being such a major issue against horde armies.
Overall a relatively weak choice from my perspective. I'd rather have 6 flamer equipped marines with a promethium pipeline for torrent than 5 flamer marines with a 2+ save and a nearly useless melta attack. Flamer marines shouldn't really be under attack from heavy weapons and should be in cover anyway so they don't really need a 5++ invulnerable save.
In the above example, you can have 6 marines with flamers with a promethium pipeline for a 4+ cover save (that will blow up on a 6+ if you make a cover save (not destroying the pipe, just doing damage)) (barricades are an option of personal preference. If you don't like cover that explodes on you drop a marine and take the barricades for a 4+ cover and difficult terrain) all with Torrent on their flamers. Or, you can take 5 marines with a 2+/5++ equipped with normal flamers and a 6" Str6 melta attack.
Personally, I’d be taking the pipeline squad as they have Scoring and do not take up a Heavy Support Slot
Moving on to one of the best units in the from all heresy forces (I think), Firedrake Terminators!
Terminators with 2 wounds and Cataphractii pattern armour for a glorious 4++ invulnerable save. Already pretty appealing. Add to that Implacable Advance (so Scoring) and you have a pretty awesome unit.
With the option of Powerfists, Chainfists and Thunderhammers, these guys are perfect for squashing that annoying little 2+ save, multi-wound FnP death star your opponent keeps bringing.
With concussive Thunder Hammers, Firedrakes make for good Primarch or Monster hunters. With Chainfists, drakes make for excellent vehicle hunters, and with ap2 and str8 they are more than able to deal with whatever payload that Spartan Assault Tank was carrying.
As if this all wasn't enough, these guys can get, Drumroll........... Storm Shields! That's right, these guys can get the dreaded 3++ save.
Charging out of their own Spartan tank with Stormshields and Chainfists/Thunderhammers (based on opposing list) you can be sure they'll shred whatever they came to shred and you can bet that your opponents entire list will swing around to kill them, freeing the rest of your list from fire for at least a turn.
Another option I particularly like is giving these guys a Kharybdis Drop pod, this 20 man drop pod can carry a full squad or 9 and someone else (I’ll come back to that), Deep Strike in, move as a flyer, do damage to everyone near it with its Fire blast attack. Then you sink like hell for the enemy turn (they’ll be hitting on 6’s as its a flyer), fly up on your turn and drop out the payload on top of whatever poor target you selected.
As this unit is quite expensive you’ll want to have them going after priority targets, not generic joe squads. Drive or drop these guys into the hardest part of the enemy (timing can be important with the pod, you’ll want at least 1 other pod so it does come in turn one when it's isolated), rip and tear the expensive core of the enemy army while the rest of your list targets the weakest sections wolf pack style. This gives your opponent a nearly impossible choice; ignore the giant 2 wound terminators with str8 ap2 running amok in their lines and defend their dying support and scoring units or, take out the Terminators but sacrifice their support and scoring units.
In an ideal world this can cause castle ups, which is awesome as you can spring from assault to assault without having to footslog, and, it gives you near total control of the mid-field and your deployment zone. Meaning you’re free to capture mission objectives and score points while your opponent is boxed into their starting area.
The drop pod version of this tactic is more difficult and relies on a highly mobile force that can ensure that the terminators aren’t isolated. Because of this I recommend a full drop pod army, with either light forces coming in turn one to establish a beachhead and some rear board presence before the heavy units come in, or heavy units come in turn one, charge turn 2, allowing your light units to drop in and score without the fear of destruction.
The favourite nasty tactic for Firedrakes is to attach a Primus Medicae to them inside their transport. This adds an extra layer of protection in the form of 5+++ FnP on top of their armoured transport, their 2+ armour and their 2 wounds. Additionally, with to the new space marine powers (which at the time of writing are HH legal) you can add a Librarian rolling on Geokinesis for Earth Blood. This gives the unit ITWND and replenishes the wounds of a nearby character (Lib or Firedrake Master). By adding either (or both if you’re a real jerk ;D) you can dramatically increase the durability of this squad to stupid levels.
Finally, they can be taken as a bodyguard unit for Vulkan, which gives you an extra slot (as this bodyguard unit exists outside the FOC)
Overall; this unit is one of the strongest Legion Specific units in the Horus Heresy Expansion and one of the most versatile assault units. With options for both anti TEQ/Monster/Primarch or anti-Vehicle war gear, this unit can bring the pain of any expensive enemy unit (and if you’re playing Alpha Legion it’s defiantly worth a steal through Coils of the Hydra). While this unit needs a transport, it is one of the strongest units and capable of easily earning it points back when pointed at the right targets.
Salamanders’ special units are a (50/50) mix of pretty garbage and amazing. Their Pyroclasts are equally gimmicky as Word Bearer Ashen Circle and their Terminators are on par or better than Gal Vorbak, depending on play style and use.
I would strongly encourage you to look at fitting in Firedrakes into your army if and where you can. Pyroclasts, not so much.
That's all from me for now, final thoughts on Salamanders are incoming, looking at both Rites and their Characters in due time.
Until then, Happy Wargaming you crazy pyromaniac space marines.