The only thing I'd say to defend F9 against thugs, is that I believe Flaming weapons still count as magic, even if the target is immune to fire. Thus Mistform and Etherealness can be countered, which helps a lot against some thugs.
The fire attack is 6 points of magical, AP damage, but the actual melee damage isn't affected. Fire-immune unit is safe with etherealness, since etherealness works just fine for the melee part and fire resistance protects him from the separate magical damage.
I'm not sure how Mistform works. Fire damage could negate it, but I'm not sure - it could go either way, really.