Tanks first appeared during the Flers-Courcelette battle. They were very slow, never going over 4 miles an hour which was bad for the obvious reason that, if they have surprise attack happen, they aren't able to drive away quickly. The people inside would have to get out. That alone is a huge disadvantage, however one of they key reasons why tanks were created, was to be able to get through barbed wire by using grappling hooks or flattening out the area so that soldiers could walk over. Another big disadvantage was that tanks were extremely hard to work and maneuver, you have to remember how much these things weighed and the ground was uneven and muddy. Being inside a tank wasn't very pleasant either, it was very hot and cramped.
During world war 1, 10 different styles of tanks came about. Some much faster and easier to work. In my opinion, the biggest positive thing about tanks is that they are scary. Tanks were understandably intimidating. Another pro is that they couldn't be destroyed with a machine gun, meaning it took powerful guns to destroy. Tanks made a lasting affect on the world, as they are still used today.