A UPF 15–29 balaclava blocks 93–96% of UV radiation, which is moderate protection that works fine for certain skiing situations. It's a solid choice if you ski occasionally, stick to lower-altitude resorts, or mostly hit the slopes during midwinter when overcast skies and short days limit your sun exposure.
The catch is that snow reflects up to 80% of UV rays, so you're getting hit from above and below. For full days at higher altitude or during sunny spring conditions, UPF 15–29 may leave you vulnerable. You'll definitely want to apply sunscreen to any exposed skin—especially around the goggle gap, nose, and neck.
If you regularly ski at elevation, have fair or sun-sensitive skin, or spend long full days on the mountain, stepping up to UPF 50+ is worth it. But for casual resort days or night skiing, a UPF 15–29 balaclava paired with sunscreen gets the job done.