Proposed new title: Heat Metal Spell. Unfair to casters when there are never any worthwhile foes wearing metal armor?
Literally I took it at 3rd for my Bard, and swapped it out at 7th never having cast it once. Many combatts are against beasts, or a dragon, or whomever where there is not a single heavy armor wearing target. Then you have combats with a bunch of brigands or whatever, but Concentration means that I can either cast a spell that will affect a bunch of foes like Hypnotic Pattern, or I can cast a single target spell that precludes me having up any other concentration spell, and will likely be dead soon.
For four whole levels, Heat Metal was never the right choice to cast. Part of that is that I was comparing it to the rest of the Bard spell list, which is chock full of great Concentration spells, but even there damage spells do stand out because there are so few of them.
So it's a very situational spell. Quite good against a single target wearing metal. But less effective than other Concentration spells against a sizable force with metal armor, and completely useless against non-metal-users like beasts.