What are the disadvantages of DC motor?
It depends entirely on the motor design. The only universal disadvantage of a DC motor (vs. an AC motor) is the need for a DC power supply (DC power is more difficult to transmit efficiently over long distances, hence the use of AC for typical power grids).
Typical brushed DC motors (the cheapest and hence one of the most common type of DC motor) tend to wear out eventually due to friction and draw MUCH higher current when stalled than when turning, which can cause overheating. They are also typically heavier than AC motors with similar specs. Of course all of these problems can be eliminated by using a (slightly more complex) brush-less motor design.