What Pyromancy rules are you looking at? BC has them in the Tome of Fate, and they're hardly parlor tricks...it's a total one-trick combat pony, but potent ranged weapons, and Immolation is just ridiculous if you land it on a BB(G)G.
The ones in BC are woefully inferior to any other class of psychic power offensively, and are good for nothing other than setting armourless mooks on fire. which dont get me wrong, is worth a giggle and half, but a dedicated pyromancer would lag behind not just other psykers but regular characters with heavy flamers, and possibly regular flamers too depending on talents and exotic ammunition.
For comparison (and this is just one of many examples);
Fire Storm is a Storm-type that requires a -10 focus test, does 1d10+PR Pen 0 and if you score 3+ DoS, you set the target on fire. A Heavy Flamer does 1d10+8 Pen 5, doesnt require a test on your part to shoot and sets anyone who doesnt pass a Agi test on fire and hits everything in a cone.
So, even putting aside the hazards of psychic powers, a heavy flamer is more likely to set a enemy on fire, more likely to hit lots of enemies, more likely to damage armoured foes and has slightly more reliable damage.
And on the psychic side, we have Force Storm, for the telekines, costs 150 less exp than Fire Storm, and does 1d10+(3xPR) Pen 0. So it literally does straight up better damage for less exp. Cant set people on fire, but its generally more effective.
Putting that aside, Pyro also has alot of werid powers, like Molten Man, which no one will ever use, because it destroys all your gear, and Manipulate Flame, which is barely better than a minor power from previous lines. Theres also the fact that, unless something is alreayd on fire you have to maintain one power just to use all the others. I could keep going.
The consensus is, that pyromancy is a weak discipline compared to unaligned and telekinetic powers, and lacks the utility of divination, biomancy and the aligned powers. Its use comes in twofold; the first is indirect fire. pyromancy attacks originate from the source of flame, if theres a burning wreck on the other side of the enemy's cover, guess whos getting mindburned? The second is in action economy, most pyromancy powers are free actions
, they do have the Attack type, so you cannot attack more than once with them, but you can do a full action worth of other things, such as maintaining, moving, casting non-attack powers, etc, and still attack.
Combined, these make for some very interesting uses in the hands of a skilled player. But ultimately, the entire disciplined is intended for combination/weird situational use, not a main focus. The book bookends the Pyromancy dicipline with these bits; "Many disdain the manipulation of fire as a crude and limited psychic discipline, beneath the notice of a true practitioner of the sorcerer’s arts. While this may be true
, the ability to incinerate one’s enemies with a gesture does generate a respect all its own." and "Of course, there are those sorcerers who count pyromancy as merely one tool amongst many in their arsenal. These individuals are often amongst the most dangerous of foes"
Now, if you wanna play around with that, thats your preference, but in OW the Pyromancy discipline is more.... elegantly straight forward and useful, you have the same three bolt/barrage/storm powers, but they all are direct fire/dont need a source of flame/have the Flame quality/do abit more damage, and in addition to those, they have a Blast power too, a high pen melta-ray, as well as a self immolation spell that is more or less a straight upgrade of BC's flaming gauntlets, and a fire-shield.
I just figured id point all this out since i adore pyromancy myself and was often disappointed in its effectiveness in the game, ever since OW came out (and DH2 has the same powers) ive switched over to the new rules and never looked back. Abit 'o wisdom of the ancients for your pyromancer