Drum brakes can lock wheels. Locked-on brake applications (parking brake) tend to be easier to implement with drum brakes than with disc brakes. Being able to lock wheels, with either disc or drum, is primarily a question of effective brake radius, applied brake shoe/pad force, and friction material.
Disc brakes have several advantages over drum brakes, lighter, cool off faster, easier to implement anti-skid control, have a more progressive or linear feel (pedal pressure vs. braking force).
Locking up the wheels, once, is largely a question of how much force is applied to the rotor by the brake pads. Sizing brakes for general use is far more involved than just being able to lock the wheels. Still, determining the force/torque needed to lock the wheels is a question of adhesion, wheel diameter, vehicle weight, rotor or drum diameter, and how much force can be applied to the brakes (how strong is your leg). Typically for maximum braking capability, an adhesion value of 1 is often used.