It's more a question of how much momentum you need to dump that determines if they are practical or not - magnetorquers are rarely used for direct attitude control, since they don't produce much torque. Instead, they are used "dump" excess momentum stored in the spacecraft's reaction wheels. The wheels can generate high torque, but they do so by changing their rotational speed, storing angular momentum. Ultimately, wheels have a finite maximum rotation speed, so they need to be "desaturated" by transferring angular momentum elsewhere. Magnetorquers provide a way to slowly transfer the angular momentum to the earth over time via its magnetic field.

They're sometimes used on small/micro sats that don't have any reaction wheels or CMGs.

Solar sailing (pitching the solar panels) is another (and more "free") way to dump momentum by using the solar wind. Obviously you need a big solar array for this to be practical.