The disadvantages of a unitary system include a lack of power balances, the possibility for slow national response because the national government must control everything, the lack of power in the local government and a lack of representation among its citizens. The British government is a unitary system and the United States government is a federalist system.
A lack of power balance occurs in the unitary system because all of the power is placed in the centralized government. This centralized government might choose to give power to local governments, but they always have the choice to withdraw that power as well as increase it. There is also not an area of the government that oversees the power in the central government. Unlike the United State's federalism system, which includes three branches — judicial, legislative and executive — the unitary system allows the centralized government to make all of the decisions for the country.
There is also a lack of power among the citizens living in the unitary country because they do not have the option to take action at lower levels, such as at the city level. In the United States, a citizen would be able to run for Mayor and would be able to be an active participant by voting for action at the local and state level as well as at the national level. The unitary countries can be either constitutional democracies, as Britain is, or can be "unfree non-democracies," as Libya is.