But, again, KM, those threats you talk about in the jungle campaign - disease, poison namely - only really bother a low level party anyway. After 5th level, all of those issues vanish. Delay poison, create food and water, Lesser Restoration, Cure Disease, poof all jungle problems gone.
Now we get into kind of weird territoy, because you're generally right (with the exception of poison -- you pretty much always take at least an initial hit from poison, even when you've got delay and neutralize), in a broad sense. This shows that the game wasn't really designed for a tropical setting, and while this is fine in a standard medieval melieu, in the tropics you're going to want to make such things a stronger threat for even a high-level party.
So you introduce things like poisons and diseases with SR (like Nyambe, set in Africa, did).
Now, for most PC's, this helps solve the problem -- the troubles are still a threat, even at high levels, because now magic doesn't just poof it away (though it does help you fight it).
Some of your specifics are off, because food and water I didn't really point out as being a problem (they rarely are, especially in a jungle teeming with life), and lesser restoration helps with fatigue, but the weather and walking around aren't something high-level adventurers worry about, anyway.
But even in a jungle campaign without house rules for these things, there's a one key difference between the warforged being immune and the rest of the party gaining immunity through spells: the people casting spells have
earned their immunity, through gaining levels and ascribing one of the most precious rescources (PC levels) to things that would gain them immunity (at the cost of some of their rescources).
What does it "cost" the warforged for his immunities? From the racial abilities, it looks like it costs him full healing and ability penalties like a half-orc. The healing is the most significant, but it's a pain in the butt for the entire party, and, again, mandates a special consideration with a special solution (the repair spells). So the cost of playing the warforged is actually to make OTHER characters use up their rescources.
So, if after 5th level, these aren't a problem, shouldn't the warforged have an ECL of +4? Because the other party member have sunk 5 levels worth of XP into gaining these immunities, why does the warforged get them cheaper? I mean, after all, an ogre isn't given his ECL based on what he'd be like as a wizard, he's given it based on what he'd be like as a barbarian, at his peak effectiveness. At their peak of effectiveness, warforged seem to AT LEAST warrant an ECL of +4, by WotC's own rules, and by your evidence here. That's part of what ECL does, after all -- keeps the valuable high-level (or mid-level) powers at a level where they belong. What are they doing running around with 1st-level parties?

5 levels is a lot of adventuring time, after all...nearly half a year sunk into your characters before they get to that point.
Now, again, it should be noted that I'd prefer a solution that made them balanced for first-level parties, such as by them gaining full healing, removing the immunities, and gaining hefty (+4-ish) bounses against these effects instead. They would still be (arguably) one of the strongest PC races (compared to a dwarf), but they'd at least be welcome in a first-level jungle...where, the claim is, they're welcome now (though they're not really going to be appreciating why it's special).
Storm Raven said:
I would look at this as an opportunity as a DM though. Even though the warforged is immune to a lot of the challeneges that "fleshy" characters might face in the jungle, he is more susceptible to other jungle specific threats that his companions are able to much more easily overcome. The constant humidity is likley to play havoc on his metal body. His inability to swim is magnified in an environment with swampy rivers and quicksand. His increased weight makes relying on rickety bridges and hanging vines for support in difficult terrain a hazardous affair. Consequently, though the warforged may be able to come to the rescue of his human companions for several jungle hazards, they will have to come to his aid for others, meaning that the party needs to work together to get through the jungle successfully.
Yes, you can always create special circumstances in which the warforged are uniquely challenged, but this just points out their exceptional status all the more. Like in a grim-n-gritty campaign with Farty McCrablegs, you have to always treat Farty in an exceptional manner because he plays pranks on the people who sell water and he throws pies instead of preserving them for his next meal, relishing the sweet taste.
In other words, the need to cater to them sperately can have an effect on the mood of the entire campaign, making them largely inappropriate, not because they have special extreme strengths, but because they have special extreme weaknesses.