I kind of despise heat metal. Depending on how you read it, not only is it ongoing damage it's the disadvantage. It's not like a fighter can drop their armor.
So instead of the BBEG being a credible threat, the bard casts heat metal, runs away, nerfs BBEG for the rest of the fight. It's so detrimental that I never use it on PCs.
There's also other issues I have with it. If you're wearing metal armor you are not in direct contact with it, you are wearing a gambeson underneath. It's also not like armor is one solid chunk of metal.
So I grant a con save every round, if you make the save you take no damage an don't have disadvantage.