1. The disadvantages of friction are as follows.

(a) Slows down or stops the movement of objects
A bigger force is needed to overcome the friction so that an object can move faster.

For example, a bicycle will eventually stop if it not cycled consistently. if you want make the bicycle speed up, you have to cycle it faster. That means more energy is needed to overcome the frictional force.

(b) Causes the surface of an object to wear out - for example, soles of shoes and surfaces of tyres.
(c) Produces unnecessary heat - Car engines becomes hotter because of friction.