The bonus is, what, 50%? Even compensating for that, it still has a vastly larger hit point pool than a mobile mortar, and takes much longer to kill than mobile mortars ever would, even not taking brace into account.
Sorry, even with that factor, its survivability is still greatly in the plus, not a minus as you are trying to make it seem.
IIRC and unless they changed it, flame weapon damage stacks 3 times against british emplacements.
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