Like many others, I am comparing the Nuclei to the gold standard of synthetic insulated jacket that is the Atom LT. A bit of background, I owned my Atom LT for 4+ years now, I am basically wearing it everyday when a little warmth and wind protection is needed. The great thing about Atom LT is you basically don't have to take it off, it hits the magical trifecta of "warm, windproof, and breathable" to a level that is great for 3 season use. My main activities are hiking, backpacking, and climbing.
Comparing between Nuclei and Atom LT, the biggest difference is in the arm pit area, Atom LT has the power stretch fleece material, while Nuclei is nylon fabric throughout. This makes Nuclei warmer, perhaps a little lighter, but not as breathable as Atom LT. I went snowshoeing last weekend in 35-45F, no wind weather, wearing a long sleeve base layer, a thin First Ascent fleece hoody, and Nuclei. Immediately I overheated on the uphill section (2000 ft. in 3 miles)and have to take Nuclei off. Because of its all nylon construction, there is no exit for the sweat vapor to go. I can see this as a good thing on a windy day, but if you tend to sweat a lot when moving, this can cause problems. My synthetic base layer stay wet on the downhill bit because I put the Nuclei on when I got to the top. I know that if I had my Atom LT, I will at least be dry by the time I got back to the trail head.
I plan to use my Nuclei for belaying and hiking on windy days, and as a cold weather around-town piece. It is still a wonderfully crafted jacket, but in my opinion not as versatile as the Atom LT.