Men's-specific ski base layers are cut to match male proportions, with broader shoulders, a longer torso, narrower hips, and a front fly on the bottoms. These aren't minor details—when you're layering up for a full day on the mountain, a proper fit means the fabric stays flush against your skin to wick sweat effectively, with no bunching or sagging that can create cold spots.
Unisex base layers are typically built on a men's fit block but averaged out, which can work in a pinch. However, they often fit too loosely in key areas or lack the fly convenience on bottoms. If you're spending full days riding lifts or hiking for turns, the tailored fit of a men's-specific piece will keep you more comfortable and better layered.
The bottom line: for serious skiing or snowboarding, go with men's-specific. Save unisex for casual resort days or if it's your only option.