Mittens are the warmest ski handwear option because your fingers stay together in one compartment and share body heat, rather than being isolated like in gloves. If you regularly deal with cold hands or ski in bitter conditions, mittens can be a game-changer for comfort on the mountain.
The tradeoff is reduced dexterity — tasks like adjusting goggles or zipping jackets take more effort, and pole grip isn't as precise as with gloves. That said, most skiers adapt quickly, and mittens work perfectly fine for normal resort skiing and pole use. You don't need finger separation to hold a pole securely.
If you want near-mitten warmth but better pole control, consider a 3-finger (lobster) or trigger mitten design. These give you an isolated index finger for grip while keeping your other fingers together for warmth. But if cold hands are your main struggle, straight mittens are tough to beat.