Read the spell description.
Sure, 4d8 in itself is not much, of course. 4d8 per round as a bonus action for 10 rounds with a single spell slot, however, is a lot, especially at a level where a paladin only has second level smite slots. Which is why upcasting heat metal can be extremely powerful. (And it gives disadvantage for those 10 rounds, and there’s no saving throw or attack roll involved. If you’re wearing metal armor, it will fuck you up. And if you happen to be resistant or immune to fire, well, that bard 3 / sorc 4 might have transmuted spell.)
I know the spell. I told you. Two attacks with a longsword from one hand will do more damage than that. I’m talking about a bland longsword and no ability whatsoever on top.
On average, an ability modifier of +4 is about a d8, and +5 is about a d10. 2 longsword attacks make 4d8. 2 2hander attacks make about 5d8. That’s without any resource invested.
You’re telling me that your awesome spell requires a lvl4 spell slot that you have 2 in the day to do less than any lvl5 martial would do without expanding any resources. And you need your enemy specifically to wear a metal armor, or your spell does about nothing.