What are the disadvantages of privatization ?

By privatization, I assume you are referring to the handing off of governmental functions to private purveyors, for example, the private prisons that now sometimes house sentenced criminals.  The disadvantages of privatization flow from the fact that, in a capitalist economy such as we have in the United States, private entities are generally for-profit and thus have a completely different mission from the government.  A for-profit company exists to maximize profit.  A government exists to carry out government functions such as educating its citizenry, distributing various benefits to create a safety net,  and providing safety and security.  These missions are quite often at odds with one another.  Let's look at two examples. 

The privatization of prisons has for the most part meant that prisoners are not properly cared for and that there is an incentive to encourage more people to be imprisoned.  The government, as it imprisons people, is responsible for not overcrowding, for proper meals, and for medical care, at the very least. Less could be considered cruel and unusual punishment.  When the government runs its prisons, it takes care to do so properly.  The buck stops there.  But when a private entity runs the prisons, it wants to make as much money as possible, sometimes accomplishing this by cutting back on food or medical care and sometimes not having enough staff. There may very well be government inspections of the facilities from time to time, but in between, these places are inclined to do their very worst, so they can make more money.  Furthermore, the government has no particular incentive to imprison more people, but the private prisons do.  There was a horrific situation in Pennsylvania in which at least one judge was getting kickbacks from a privately run prison camp for juveniles, for those he sentenced to the camp, whether or not they were guilty.  Situations like this cannot exist when a prison is government-run. 

The disadvantages for a privately-run school are the same as they are for prisons.  I am speaking now of K-12 education, which should be offered everywhere by government.  We now have many charter schools that are for-profit institutions, and again, their bottom line is far more important to them and their investors than the education of children.  These are particularly reprehensible, since each student who is attending a privately-run charter school is being paid for by the taxpayers, taking funds away from the children left behind in the public schools and helping these private schools profit.  There are those who have argued that private charter schools are better at educating students, but there is little or no evidence over the years to demonstrate this.  When government is educating students, its mission is to educate everyone to be a citizen who participates intelligently and critically in a democracy. It may not always succeed, for a myriad of socioeconomic reasons that I could write a book about, but at the very least, we need not worry about its focusing on profits. 

Privatization might work for some kinds of situations in which the work being done is not an essential governmental function, for example, providing a swimming pool. But I am very skeptical about privatization generally, since its focus is always making money, not doing what the government is supposed to do.