If there is a unitarian government, then there is no greater power than the federal government -- nothing can stop their legislation, no even the state governments. Also, consider that combined with an ill-suited leadership spearheading the unitarian government. If the unitarian government is supreme to all, and the people that control the unitarian government are corrupt, incompetent, stupid, ignorant or ahusive or their power, then there is no force that can stop them from doing what they want. No (powerful) state governments to keep the federal government in check.
Consider a situation in real life: Note that Canada, as previosuly stated, has a federal system, and the UK has a unitarian government.
If I am a Canadian citizen, and I am overweight, and I want to go make it against the law to make fun of people only because they are overweight. So, I can go to the state government to try and pass a law that will do so. If they say "We don't have the power to do that, sorry", then I can go to the federal government and try to get them to pass the law. Maybe they will or maybe they won't -- the point is, I had the OPTION to try and persuade ANOTHER level of government to pass the law.
Consider now that I am a citizen of thd UK, with the same situation and goal. If I want to pass the law, I must go to the federal (unitary) government because the state government have no power (this is true -- they exist and are disbanded at the whims of the unitarian government). If they say "No, we won't do that", then I have nowhere to go. I cannot do anything about it.
The moral of the story here is: The advantage lies in the federal system because one has the option to go to another level of government to pass legislation, and also because if one level of government has too much power, they tend to become tyrannical.