Engineers are not gunners, but they still shoot. Chem Spray may be dull for that one guy on a Pyramidion's bottom deck, but on a Junker's top deck it could be much more stressful for how "easy" it is.
For some of the proposed suggestions which imply that having both Fire Extinguisher and Chem Spray is the best solution, I argue it is not. I hope just mentioning the Mobula here is enough, but also Galleon and Spire might very well suffer as the engineers want to shoot when they are not fixing the components they are closest too instead of running across the ship to fix the other guy's component. The casual player wants to have one tool and be effective with it, it shouldn't require the other tool too. But I am probably making it sound worse than it actually is...
Regarding a less than 100% fire immunity on Chem Spray: Sure, but why. It will be balanced to a point where only very few fire stacks will be applied to the chem sprayed component to the point where it can be extinguished with Chem Spray alone. Also, the current Flamer is very different to the old Flamer, since it stacks fires insanely quickly, it might just eat straight through the non-perfect fire immunity while other fire guns don't propose any threat when compared. (Is that actually good?)
A problem that both anti-fire tools share is definetely that they are both very similar and don't have that many cases where they are applied. Mallet and Spanner at least have Repair and Rebuild, but the anti-fire tools just have extinguish while interrupting repair cycle or use before fire to get some fire immunity for a sec. I am making sense here, right?
My point is: No matter what, as far as I can tell if one of these tools don't change their mechanic, there will always be one superior anti-fire tool, which is Chem Spray for the moment as it can actually effectively fight fires with a minimul interference to the repair cycle. For example, what I am thinking about is Chem Spray replacing the buff when applied, whereas the Fire Extinguisher changes to something very similar to the current Chem Spray. Ultimate fire immunity at the cost of less survivabily and manouverbility or the best from both worlds, which will constantly interfere with the repair cycles. This way you could have the same tool twice or combine both of them to minimize the disadvantages for different playstyles, because, for example on the Pyramidion, you may want buff on the balloon so you take a Fire Ext, while on the Hull and Engines you take the Chem Spray so your repair efficiency is maximized. (A thing to note here is that this version of the Chem Spray won't hold while a component is destroyed, while this version of the Fire Extinguisher will.)