What are the disadvantages and advantages of alloys?

Advantages of alloys:
-high strength-to-weight ratios
-a wide range of different alloys, to suit a range of different uses 
-low density

Disadvantages of alloys:
-No fatigue limit
-prone to corrosion, so need protective finishes, particularly magnesium alloys
-Many alloys have limited strength, especially at elevated temperatures magnesium alloys have low strength

They are all decomposition reactions.