Protectorate's character 'jacks are, almost without fail, very solid pieces. Competitively priced, make great MAT 7-8 options, and almost all have some kind of strong, unique role to fill. Fire of Salvation is, in my opinion, the worst character 'jack in Protectorate... and he's still great. I've fielded him quite a bit, especially earlier on in the edition change, and I just want to go over a bit of exactly what he brings to the table.
It's not so bad, being the worst of some great options.
Fire of Salvation is Kreoss's warjack, and he gets across the Intercessor's fervent hatred for his enemies and their spells. Although you could say he's "the" character Crusader, he's fairly different in a couple of important respects, the main one on the base stats probably being that he is SPD 5, which keeps up with the average Warjack in this day and age. He's MAT 7, which one would expect of a character, and from there he's largely quite similar to a Crusader. DEF 10, ARM 19, two 1" melee attacks at PS 14 and 18, although rather than the Crusader's crit fire on his Inferno Mace, Fire of Salvation's Absolver always causes continuous fire.
The back of Salvation's card is where he really starts to pull away from the blank, vanilla Crusader, and is where he really starts to earn that extra 6 points. Firstly, this Warjack belongs to Kreoss, and if you take him with Kreoss his weapons all gain Dispel. Outright Dispel is a great effect, and makes it so very few spell effects will stop him from crushing foes of the Protectorate. That said, the ability to get around effects like that are becoming more and more common every day it feels like in the Protectorate, but it's a welcome ability nonetheless.
Next down is his Imprint: Holy Fervor. Now, this ability would be truly insane on a model with a 2" melee weapon. On Salvation, though, it's merely pretty good. For the cost of a single focus at any time during its activation, all of his attack rolls for the rest of his activation become boosted. If you're at all concerned about your ability to hit a target, this Imprint is pretty incredible. Pop it before you go in, you lose an attack but you'll be hitting DEF 17-18 models on average dice. Against low armor, high defense pieces like Warpwolves or a Warcaster, it's extremely effective.
Additionally, whenever he kills an enemy model with an attack, he can immediately attack another enemy model. This has none of the downsides of berserk really, you get to decide if you make the attack or not so you aren't going to start killing your own stuff. This makes him pretty efficient and just clearing a few infantry models in melee (or cleaning up nearby infantry after you kill a heavy) as you can basically kill every infantry model you get into melee range with for only a single focus... which conveniently he gets through power up.
The final rule on Fire of Salvation is Righteous Vengeance, and this to me is where he really gets an advantage over most of our other warjacks. If an enemy kills a Friendly Faction warrior within 5" of Fire of Salvation in the last round, at the start of your turn Salvation can make a 3" advance and an attack.
This gives Fire of Salvation an enormous threat range for a 'jack that is only SPD 5 with 1" melee. Stack that with any other major threat extension, and he can start threatening ridiculous distances, sometimes with quite unpredictable angles. On his own, with Righteous Vengeance procced, he will threaten 12". With Assail on Intercessor Kreoss, he gains focus efficiency and goes to 14". High Executioner Reznik with Perdition can push that up to 15". He just has that couple extra inches of threat that is fairly unique to him.
So who do we take him with? Here's a few of my favorite spots.
Thyra, Flame of Sorrow
This is my favorite spot for him so far, and where I've easily played him the most. Paired alongside Blood of Martyrs with a list largely otherwise running infantry, you have two 'jacks with almost completely impossible to predict threat angles until after he feat has been used. You get two ridiculous accurate warjacks that can get to crazy damage values and threaten large distances, mostly at the cost of ranged presence which you can get back with something like Idrians. He adds to an out of activation movement skew I've used with Thyra for some time now, and I've loved him here.
High Executioner Servath Reznik
This is mostly just for the threat range. Paired with Scourge of Heresy, Reznik can perdition something, kill a model, and if Righteous Vengeance was activated Salvation gets the additional 3"... you have two jacks going for miles. That said... Reznik has other issues.
You don't get as much out of Dispel necessarily since you have a feat that strips upkeeps, but it's nice for other turns. What you really want here is Assail, which gives free charges and more charge distance (or slam, if that's your thing.) The free charge makes the Imprint more cost efficient; think of it almost as a free imprint with your charge instead, unless you're attacking something pretty low DEF or don't need the berserk-effect. Plus you can take him in theme. I'm not sure I'd take him with the other two iterations of Kreoss... possibly Grand Exemplar but really I don't have much experience with him.
There are a few other places I could see thinking about him (Harbinger, Durst I suppose) where he's just a decent piece for the threat range increase but.... overall, I feel there's other, better options for a lot of those casters.... and truthfully, there often are with the ones I have listed above. Thyra is probably the main place I've found a home for him, but sadly as themes are on the rise, character 'jacks become much more difficult to slot into lists unless they have a real home with their bonded caster... and character 'jacks with no guns even more so.
Fire of Salvation is a strong beatstick piece, and definitely has his place, but won't be a go to for most builds, as outside their bonded casters, 'jacks like Scourge of Heresy, Blood of Martyrs, or even just good old Reckoners will often be an easier pick for the points. I think the place to use him is anywhere you can abuse his unpredictable, lengthy threat range over anything else, as that and his imprint are really what make him unique among Protectorate's 'jacks.
Fire of Salvation is a great piece that is just a bit overshadowed most of the time. In my opinion, as I said, he's the worst character 'jack in our arsenal... but he still has some solid places and is great at his job as just a hard hitting beater.