For Huayna Potosi, I climbed in Millet Rock&Ice boots, which I regularly use for summer mountaineering in the European Alps. I've dabbled a bit with ice climbing in the winter with them as well. They were fine for both jobs, never had cold feet. Since winter in the mountains in the northeast of the US is colder than summer in the Alps, your boots should be fine too.
On the higher peaks, including Illimani, I climbed with Scarpa Omega plastic boots. Overall, they were fine, but especially on Parinacota I had cold feet during the night. Nothing dangerous, but a little bit more warmth would have been nice. Mind you, I only wore one pair of socks in them - the fit is too snug to wear two pairs. Perhaps I should have bought them one size bigger.
Last year I climbed a bunch of 6000-ers in Peru with my Scarpas, and on several occasions I had cold feet again in the early hours before sunrise. I wasn't surprised, but I can handle the minor discomfort, because as soon as the sun comes out, it's fine again, and these boots have a lot going for them: they are light, relatively comfortable to walk in, and excellent for climbing something steep.
Based on all this, I would say that these are good for South America (especially if I had them slightly larger), but I would never take them to Denali. And in case you find yourself spending the night in an unplanned snow cave high on, say, a 6000-er in the Cordillera Blanca, without anything but your boots to keep your feet warm, you might get frostbite.