Federalism makes the state weak because there is always a conflict going on between the center and the federating units and as a result of this both the federal government and the federating units suffer. This also results in delays and inefficiency and leads to the weakness of the state.
A federation is an agreement between two or more sovereign states to create a new state in which each will exercise specific powers. This agreement is in the shape of the constitution. The constitution defines and explains the powers and the jurisdiction of each government.
Under federalism there is both unity as well as diversity. In other words it means that there is one country in which the provinces or the federating units maintain their individuality. A federation maintains a balance between centripetal and centrifugal forces.