Disadvantages
The main disadvantage of the system is that it needs a special motor. Usually motors are built to achieve maximum torque at high rotational speeds, usually 1500 or 3000rpm. While this is useful for many applications (such as an electric fan), other mechanisms need a relatively high torque at very low speeds, such as a phonograph turntable, which needs a constant (and very precise) 33⅓ rpm or 45 rpm. A 1500 rpm motor couldn't even start rotating if fed the necessary voltage to make it spin at 33 rpm, and it wouldn't be precise keeping that speed.
The slow motor also needs to be physically larger than its faster counterpart. For example, in a belt-coupled turntable, the motor diameter is about 1 inch (2.5 cm). On a direct-drive turntable, the motor is about 4" (10 cm).
Also, direct-drive mechanisms need a more precise control mechanism. Low voltage variations on a high-speed motor, which is reduced to low rpms can go unnoticed, but in a direct-drive, those variations are directly reflected on the rotational speed.
Read more about this topic: Direct Drive Mechanism