So first up on a vaguely related note. Yep Dwarfs can set stuff on fire but we don't have the intentional stuff with us, still distilled alcohol should be effective. (I was actually looking for the quote about sigmar interaction with burning shadows but gave up, have a consolation prize)
The dwarves do have alchemical mixes capable of creating intense fires, but those are weaponized more directly by spewing them onto the enemy and there aren't any of those weapons in the expedition anyway. Burning Shadows as a spell is finicky to use at any level above weaponized shadow-puppets with a torch in a dark place, it just has too many variables that are out of your control.
Now onto the meat.
disappointing, that we're throwing away the most effective option to go with burning stuff, if the burning shadows option was dangerous then the set everything on fire option is even more dangerous as you're going to be making our shadow spells all but useless, the concern that goblins could just shoot Mathilde are significantly worse in the set things on fire plan given it would require us to actually be in the Grobi town.
Honestly I don't have anything against a massively awesome casting of Burning Shadows. We've got magic 7 and our seed to back us up plus prep time to do it right. However the biggest problem with plan burning shadows is that the actual plan seems to be kind of a mess. (also I don't like the rushing about, I'm still adamant on taking our time and doing stuff right but that's a different issue)
[] Plan Burning Shadows
-[] Spend a night before the assault
hunting down and assassinating any surviving shamans1. Make sure to sleep the day before.
-[] Mathilde infiltrates into position to be down sun of the Citadel
and arranges for the dwarves to attack at the local dawn2, when the sun becomes visible above the eastern peaks. She then times the conclusion of a casting of Burning Shadows to complete moments after the sun is visible and the Citadel's shadow is cast. Ritual cast if Mathilde is confident she can do so reliably.
-[]
Scatter Black Lotus poisoned caltrops on the path to delay greenskins3 and get them stuck in the Burning Shadow longer.
-[] Organise to have indirect fire of dwarven catapults firing whatever the engineers think will cause the most havoc arcing over the Citadel into Grobi down timed to a few moment after the Burning Shadows is cast.
For example I'm not even sure if this plan includes attacking the Citadel. It seems to be all about causing general havoc on the Greenskins and I'm not sure if that's intentional or just oversight. It would help if the plan had some general info under it but I've checked the posts of everyone who voted for it but the original poster doesn't have any explanation in their post. Other questions I've got.
1: Shamans where? in grobi town? in the Citadel? -I think it's split between both but not sure
2: Are the Dwarfs attacking grobi town? attacking the Citadel? Are they marching in or standing back behind fortifications and unleashing artillery? - I think this is marching on the Citadel, but i'm not sure.
3: who's doing this scattering? Our Monstrous Cavalry makes sense since they have movement for it. Or maybe Rangers since they could do it sneakily in advance? - I think it might just be Mathilde on the immediate path to her, but I'm not sure.
We're all on the same team here and I'm not bringing this stuff up because I want to knock down your sand castle and evilly laugh over it but because I want the plan's flaws to be fixed or a convincing explanation given that proves there are no flaws in the first place and I'm mistaken.