Traditionally, chimneys are either unlined, or lined with clay tiles that break, crack, and weather with time. These conditions allow heat and fumes to transfer or escape into the home, sometimes with lethal consequences.
Condensation also damages many chimneys.
Warm fumes cause condensation to form in the flues, and with years of use the moisture can destroy the brickwork or lining of the chimney. Plus chimney fires, or lighting can destroy the brickwork and lining of a chimney. That depends on the size of the chimney, the number of fireplaces and appliances connected to the chimney and other factors. Nearly any type that isn't properly fitted for its chimney. Fireplaces, wood burning stoves, and even ultra efficient gas and oil furnaces can cause chimney problems if the flue fails to vent gases quickly and efficiently.