Complete garbage in all manners. While 25% damage sounds nice, damage is less effective than skill boost and the number is as such much lower than what it seems. The condition does not sound hard to meet, which sounds attractive- but is ruined due to furnacemech's icon. Most of the cards you would want to place Furnacemech on, especially due to the scarcity of elemental icon's sub options, do not want to be hitting red targets. This means Furnacemech is essentially either on a B-grade card vs. it's preferred target due to him, or on a card that is not benefitting from his skill icon bonus.
Nonetheless, a situational pick if you have absolutely nothing better during a red boss event dungeon, or on an Elemental icon just for the skill icon bonus. However, on-icon S cards will probably do you much better.
