Efficiency: the government may be more efficient because a few people take all the decisions, with barely or any opposition, which can make any process run faster.
Order: the government can more easily impart order in society because its authority is unopposed. This can reduce crime, and improve public safety.
Disadvatanges:
Authoritarianism: a government concentrated in a few hands can become very authoritarian, to the point of violating the human rights of those who oppose it.
Totalitarianism: this type of government can become so authoritarian that it turns into a totalitarian government: one that controls every aspect of its citizens: from the people they vote for, to the way they dress.
Corruption: a government without opposition, concentrated in a few hands, can easily become corrupt, and reduce the welfare of its citizens because of that.
