DifficultiesDriving a tank can be very hard, or even impossible for someone who has not learned/been taught the proper way to operate one. These are also very hard to repair unless you are a trained tank mechanic. They are difficult to come by as well unless you are near a base hosting tank units or in the military.
The armament of a standard Main Battle Tank (MBT) consists of a single-shot high velocity cannon from callibers ranging from 75 to 150mm in size. They typically field medium-caliber machine guns that fire from the bow or coaxialy with the cannon. They also usually have a heavy machine gun in an anti-aircraft mount on the commander's copula which can also be used against ground troops.
The primary advantages of a tank are it's heavy armor, armament and it's ability to traverse nearly any kind of terrain. Tracked vehicles like tanks are also more maneuverable than wheeled vehicles, as they can turn in place, rather than requiring some form of momentum like a wheeled vehicle. The armor of most modern MBT's are capable of resisting nearly everything; there are few AT weapons capable of penetrating one frontally. Most tanks are laiden with weapons, a few offering special weapons (coaxial cannons, flamethrowers and cannon-launched missiles). Most tanks made after World War 2 also have special systems to allow their crews to survive in a NBC Environment. Against raiders, only the most heavily armed would even attempt to attack these fighting vehicles. Against zombies, they would be practically impossible to penetrate. When out of ammo, the treads can be a potent zombie killer. A tank also does not need to be mobile to be effective, as you can use it's cannon as a stationary defence against raiders. Another use a tank has is being able to crush vehicles, improvised barricades, and other obstacles, making it useful for clearing roads or making paths and farmland.
Tanks are resource-heavy vehicles. Most tanks require large amounts of fuel to operate and often require steady supply lines. Tanks also require specialized equipment to repair or replace parts. Depending on the ammunition types carried, a tank's main cannon may not be effective against zombies. AT shot or sabot is not intended for anti-infantry use, and most shells of this type will not explode. Such rounds will not prove very useful against soft targets as the shells will simply pass through legions of undead or split them apart and making them harder to find. In a fierce battle, against either zombies or humans running out of fuel and/or ammunition will leave you trapped within your vehicle, effectively becoming your tomb. Tanks offer limited crew space and visibility, and destroying the optic systems of tanks can render them virtually combat ineffective. Destroying the treads of a tank will also strand it, and requires heavy lifting gear to move and repair. Most armored vehicles require specialized training and a highly coordinated crew to avoid injury and to use to its utmost effectiveness. Finally, tanks are simply rare and one is unlikely to find one in serviceable condition, and ones that are are likely under the control of government forces or raiders. Tanks used against zombies will eventually become surrounded by them, making obtaining one more difficult.
World War Z usage
Abrams tanks were used unsuccessfully at the Battle of Yonkers. During the reclamation of territory in World War Z, tanks were only used by the US to kill rebels. The Russian Army used tanks extensively (but inefficiently) against the Undead.
28 Weeks Later
Tanks were probably effective against these Infected as they can be killed more easily. Tanks are seen once, but they are parked and are not seen fighting at all. Although the main cannon wouldn't have been used, the machine gun would have been be its primary weapon.