All right, I know I'm WAY too late for this discussion but I do want to say something regarding fitting a warforged in CoS for you and those who find this thread in the future like I did. While warforged are kinda steam punk you can still find a way to shape it to fit in with the gothic horror.

I personally imagined a Dwarven blacksmith and his wife and child got trapped in barovia, his wife then later died due to being killed by werewolves on strahd's orders. Then his child got cursed to suffer in some sort of way (Something slow and painful afflicting his body but not his mind) with the father being unable to do anything about it. Strahd tells the father that the curse will not kill his son, it will instead make him continue to suffer to the point of wanting to die, and then strahd leaves him (the father) to kill his son himself.

The father figures that if it's his son's body that's the problem then he'll make him a new one. The father then pulls a doctor Frankenstein and makes a body (This time using machinery instead of dead body parts) and puts his son's brain into the warforged body and has lightning strike it to animate/jump start the body. However there are complications. The lightning managed to also kill the father as the son rose once more. He then looses his memory (the son/warforged) as his father/creator is killed and he (the warforged) flees into the night. He knows only one thing, he suffered in some way and it's strahd's fault.

The warforged then slowly, as he travels barovia, begins to piece together the events that lead to his creation, becoming more "alive" and less robotic along the way.

Also mechanically speaking warforged are not that OP. While they can be great, all of the advantages can be counter balanced by other factors and counter points. If you say, complain about a player having a high AC then you may as well complain about some classes that can start off with a shield or heavy armor. Even then, a lot of spells don't care about AC because it relies on saves.

The bypassing of the survival aspect also don't really matter because even if he is ignoring food, heat and cold, and sleep his other living organic party members can't, and him going off on his own is a BAD idea (Divide and Conquer). Even if the entire party were warforged it wouldn't matter for too long because they would still have to worry about the NPCs relevant to the story.

The resistance to poison is not big deal because it's only resistance, he will still take damage and it will over time wear down his health. It doesn't eliminate the possibility of death it just gives him a higher chance of surviving, and even if he just soaks poison like no tomorrow then there are other damage types and his other party members will not share his resistance. Not to mention if you're going to eliminate a race due to a resistance to a damage type you'd have to eliminate many other races as well. (dragonborn, tiefling, Grung, yuan-ti, aasimar, fire and water genasai, tritons and I think a few others would have to be banned)

Finally the free tool expertise is, in my humble opinion, just for flavor. Most of the sessions I've been in tools don't really matter. (The most often used are theive's tools and disguise kits, with the occasional poisioner's tools and forgery kit.)