Slip-on sleeve knee pads are great if you want something comfortable and low-profile that fits easily under your ski pants. They pull on like a compression sleeve with no fussy straps to adjust, making them quick to put on and very comfortable for all-day wear on the mountain.
The trade-off is that they rely entirely on elastic compression to stay put, so they can slide down during aggressive skiing—especially if you're carving hard, hitting bumps, or taking falls. Once a pad shifts off your knee, you've lost your protection right when you need it most.
Slip-on sleeves are best for recreational skiers who want lightweight impact protection for casual cruising or groomed runs. If you ski aggressively, race, or spend time in the park and terrain features, look at a hybrid or strap system instead to keep your pad locked in place.