It's the same question: is it "unfair" for the GM to use abilities, spells, monsters or other game elements that are particularly effective against specific PC types or builds, especially if those types or builds are among your players' characters? That is a worthwhile discussion and not limited to heat metal.Not sure why a whataboutism is called for here? Whether or not another spell is fair or not isn't really relevant to whether this spell is fair or not, right? But hey, start a thread about fireball if you really want answers to that.