A magnetic compass is less usable near the poles partly due to magnetic dip. One of the Earth’s poles is much closer and affects the needle more strongly than the other pole. As a result, Earth’s magnetic field pulls the needle toward the ground.
Flat-Earthers claim that a compass is unusable only near Antarctica. In reality, the same also occurs near the North Pole. Magnetic dip, as observed on various locations on Earth, can only happen if the Earth is spherical.