@Willie the Duck
On the subject of studded leather. The studs or spikes are made of metal. This is the only way to make the studds to hold on the hard boiled leather. So heat metal would work as the armor has metal components to it. So has ring mail.
I recommend you the old second edition arms and equipment guide or the Palladium Compendium of ancient arms and armors. The latter has a nice treaty on hard boiled leather if I recall correctly. A shame I lost it...
You are missing my point. Nowhere in the 5e ruleset does it describe studded leather as having metal studs. If I had a player (particularly someone new to gaming with this edition) who wanted to* get 'studded leather' that included non-metallic studs, I would be hard pressed to call this any kind of bad faith arguing that this would be a reasonable thing. Ring mail also doesn't mention metal, but I'd be hard pressed to believe that the player making this argument wasn't looking for loopholes ('what did you think the rings would be made of?') compared to someone not assuming that spikes (in a world full of scary spiked things with hide as tough as armor) would be metal.
*ahead of time, perhaps after defeating a spiked enemy and needing to find an armorer who does special commissions.
Regarding historical reality, while it is possible that there may have been an example or two of cuir bouilli armor that included metal studs*, it's been generally understood that studded leather (and ringmail) are misinterpretations of artwork depicting brigandine and normal chainmail, respectively. And that's part of my reasoning for above: who am I to dismiss someone else's mental image of a mostly-fictive armor?
*of unclear benefit, as they would just telegraph a mace blow through the leather and make sword blades slide off right onto now-perforated leather.
EXACTLY.
Do you see what I'm saying now? This is only a problem if the DM wants to be a totally massive, as you put it "dick".
They need to set up an ambush on the PCs, arrange the scenario so their guy can get away (something they have 100% power over, as this is D&D, they can make up whatever scenario, however far-fetched, that allows it), and then cheese the hell out of it. The solution is: don't do that!
That is a solution to the problem, to be sure (and why I don't really see too many issues with this spell in-play). It is still rather annoying that, of all the many ways that this spell could have been written, it is written in such a way that almost anyone will read it and immediately notice this 'dick move.'
Just out of curiosity, I checked Wall of Thorns, which used to do more damage to lesser armored characters in previous editions. In 5E it does not: the damage is the same regardless of what the person is wearing. In addition, Heat Metal is MUCH more effective than the 3rd edition version, which not only did less damage overall for a shorter period, but also had its damage explicitly mitigated by cold damage. I wonder what the motivation was to book these two spells (probably two among many).
Simplicity and making the spell fit into the formula of concentration for 10 rounds that combat spells now fall into. The 3e one took place over 7 rounds (with 2-6 doing damage) and required you to take into account which round of effect you were on.
Honestly, that part of the spell I don't mind -- even if I'm a non-casual and doing this level of tracking and calculations is trivial, I get the appeal of 'this spell is up until concentration fails (or after 10 rounds which seriously you probably don't need to think about).'
What bugs me most (about this spell individually) is the lack of save/to-hit, the breadth of things at which you have disadvantage, and the general sense that it is a high-power spell balanced by it often being inapplicable from a PC side (many enemies not wearing metal armor), but several PCs concepts where the applicability is always going to be the case). The cold-damage mitigation point is an issue with each spell being a new paragraph in isolation which only interacts with other rules when specifically mentioned. I think any fix I could devise would be a lot of complexity-in-search-of-a-purpose most of the time, but I could imagine a basic set of elemental damage interaction rules to be applied 'as appropriate' would work pretty well. I'm trying to recall any systems which do that, however, and am drawing a blank.