Using microwave ovens for cooking can be unsafe, because they do not heat evenly foods. It can be dangerous when you are trying to cook raw food, because it can leave back harmful bacteria and so it will be very bad for your health.
Cooking foods in microwave ovens can reduce moisture in food, especially foods with low water content are more prone to drying out while cooking.
Because microwave oven does not turn foods brown and crisp, unlike the conventional cooking range, therefore you may not get the desired texture while cooking.
Also, reheating foods in a microwave oven many times can further leach away nutrients from them.