Ventillation
A normal compressor-based air conditioning system works by recycling the air in the room, cooling it as it passes through. An evaporative cooler only works well when it has a constant supply of fresh air from outside. Otherwise, the air in the house will get saturated with water and the cooling effect will cease. To use an evaporative cooling air conditioner, you have to either leave windows open or install vents in your roof to allow moisture to travel through your house. This can bring in excessive dust and pollen from the outside, triggering the allergies of people who suffer from them. Leaving the windows open can also be a security risk.
Water
Evaporative coolers work by evaporating water into hot, dry air, lowering the temperature of the air in the process. Running an evaporative cooler saves electricity, but it does require a constant supply of water. If you live in an area where water is very scarce, using a traditional air conditioner may actually be a more economical choice.
Climate Dependent
Normal AC units work in any climate, but evaporative coolers aren't as versatile. For an evaporative cooler to work well, it needs to operate in a dry climate. If the air is already moist, the swamp cooler will not be able to force more water into it. As a result, it will have little or no cooling effect in a moist environment.
Mold and Mildew
Evaporative coolers use wet pads, blades, nozzles or screens to disperse water into the air. These surfaces can become a breeding ground for mold, mildew and bacteria if they are not cleaned regularly. Additionally, increasing the humidity in your house can also encourage mold and mildew to grow. People who are sensitive to mold and mildew may suffer serious discomfort or illness caused by evaporative cooling.
Maintenance
Evaporative coolers need more frequent maintenance than other cooling systems. They build up salts and mineral deposits which have to be removed, particularly in hard water areas. Pads and other parts also have to be changed. If your evaporative cooler is mounted on the roof, it can be time-consuming and inconvenient to maintain it.