Forged Limb is told to risk conflict between runestmiths and Valaya cult. It was never clarified how and why it would happen. I tried to think about possible scenarios in context of dwarven culture.
Conflict would likely be caused not bye clash between runesmith and valayans - it would be driven by conflict between runesmiths.
Scenario:
- runesmith A takes commisions and gets paid handsomely for forged limbs
- runesmith B makes agreement with valayans for healing, similar to one Snorri have
Runesmith 'A' is ticked off, because 'B' is poaching his commision. For Snorri comissions are non-issue, but it's important to remember that commisions are precious for typical runesmith. So runeshmith 'A' is upset, and points that valayans are unfairly interfering with him. They are helping 'A' to poach commisions, thus interfering with remit of runesmithing guild.
Accusations of predatory market practices could be a contributing factor but I don't think it would the main thrust of any bad sentiment from the runesmiths' side. Realistically, Snorri's probably done what you describe to a tonne of younger smiths already and nobody's made a problem of it as far as we know. Like take the underway construction as an example, where Snorri put a rune tool into the hands of basically every able-bodied miner in the whole hold. What of the various journeymen who might have had a niche putting runes on pickaxes only to see demand for their services crater overnight, because Snorri "fucking" Gift Giver decided to hand out tools by the hundreds? Nobody's been showed to raise a word of complaint, which leads me to believe market competition rules maybe aren't a huge article of faith for the runesmiths' guild. Smith A in your example might be assmad but that wouldn't be enough in itself for the conclave of runelords to intervene on his behalf.
Generally, I get the feeling runesmiths are supposed by convention to keep themselves apart from dwarfs at large, as far as their work is concerned, and a smith letting people have a peek behind the curtain, even at small inconsequential things, is going to raise hackles in some quarters. Like when Snorri worked on the pure gromril smelter, he kicked out all the non-runesmith workers and did all the runework in solitude except for his apprentices. It would assuredly have been more efficient to apply runes to sections as they finished while the builders worked on other sections of the complex, instead of finishing anything except the runes first and then shutting the whole thing down for two years while the runelord did his part, so why do it like they did? Presumably because traditional runesmith secrecy extends to more than just the physical inscription of runes; things like the way individual runes interact and which rune is contributing what to the overall gromrilsmelting process is also out of bounds. The whole thing is supposed to be as much of a black box as possible to the uninitiated. Dwarfs hate inefficiency unless it is borne of tradition, to paraphrase a soulcake quote I can't be bothered to hunt down at the moment, and this particular inefficiency is covered by the tradition clause.
Now obviously Snorri hasn't been giving the valayans lectures about how the forgelimb rune works, or how it relates to the waking rune family or its likely connection to other cognition effects etc, but just the fact that they've been working closely together with him has necessarily let the priestesses pick up on some small things at the margin. Like in the bit we saw with Moira, for example, where she explains to some guy how the prosthetics can feel weird for some users, especially in a case where the old biological limb was damaged in some way. She doesn't know anything about the processes that operate the rune but she has a deeper understanding of its output than an individual prosthetic user would have, since Snorri's contracted with her to deal with a whole bunch of users and prospective users on his behalf. That peripheral knowledge would be enough in itself, I think, to make some conservative runesmiths antsy.
This problem, if it was a problem, was limited in scope as long as Snorri was the only smith engaging with the valayans in this manner. The moment he starts teaching his rune to other smiths who may or may not strike their own deals with the priestesses, things get messier. This is partly because Snorri is being a bad influence on the youth, as it were, impressing his unconventional (therefore bad) ideas on younger masters, but also because it increases the number of potential points of contact between the two cults. Every smith fraternising with the priestesses increases the risk one of them might let slip something more important, and even various inconsequential pieces of info can, cumulatively, reveal the outline of things runesmiths have kept to themselves. There's another dimension to this too, which kinda touches on your competition thing: if you want a new arm in kraka drakk, you don't go to various runesmiths asking for quotes, but rather to the valayans who will act as intermediaries setting up a deal for you. They have a lot of influence in who gets work orders and who doesn't, which comes uncomfortably close to interfering with the independence of individual smiths but also the guild as a whole. Hovering overtop all this is a big ole slippery slope argument: what if the priestesses start prodding for info about runes related to healing and shelter, so they can do their jobs better? What if Morgrim Crew gets inspired and starts fishing for their own deals on engineering runes and which devices to put them on? This is the sort of thing that can make some runelords mad at the smiths under them, but also the valayans, for knowing things they weren't supposed to know and deciding things they aren't supposed to have a say in.
This whole controversy interrelates with the broader suspicion Snorri might be collectivising, exerting undue influence on other master smiths and so on. It's a whole mess of entanglements between actors who were once tidily separate, and in the middle of it all you'll find Snorri, like a spider in a web. Anyone caremad about some or all of what I've posted about is going to find an obvious first target in him.
