What were some of the challenges in the U.S that ex-slaves faced after they were freed?
The American nation did not make provision to integrate the freed blacks into society. They were treated in the north and west, and by the nonslaveholders of the South, as a pariah.
Many or most of the northern and western states had laws forbidding free blacks from entering. These states also had laws limiting the political and economic rights and opportunities of the free blacks that already lived there. Pretty soon, the South started copying those laws. Upon being freed, the only friend the blacks had was their former owners. After the Republican party used the blacks to put itself in power in the southern states, even their former owners no longer thought well of them.
Under slavery, blacks had food clothing and shelter from cradle to grave, in sickness and in health. Under freedom, they had these needs only so long as they could work, and very many of them had not had any training or education under slavery that would help them cope with freedom; furthermore, they were subject to laws that limited their movement and what jobs they could hold and so on. It was one generation before the southern whites recovered the average economic well-being that they had know before the War Between the States. It was three generations before the southern blacks recovered the average economic well-being that they had known before the war.
The nation did not help them to homestead lands in the west, because western whites did not want them there. The nation did not break up plantations in the South to provide them land. (That would have been unjust to the owners, but the reason was that the nation wanted cotton produced on those plantations, and small farms would not have been very good for this.)
Donald, Henderson H. 1952. The Negro Freedman: Life Conditions of the American Negro in the Early Years after Emancipation. New York: Henry Schuman. I found this book in a university library near-by to my home. Your school librarian might be able to borrow it for you via Interlibrary Loan, or you may visit a nearby university library yourself. If it is a state university, it will probably let you check the book out yourself.
Livingston, Donald W. 2010. "Why the War Was Not about Slavery," Confederate Veteran, 68, 5 (September/October), 16-22 & 54-60. If you can't find a copy of this article, send me a message, and I can send it to you.
I would say that the most important challenge that the freed slaves faced was economic. They had very few opportunities for making a living. Most specifically, most slaves' only skill was agricultural. So it made sense for them to become farmers but they had no land and no money to use to buy the seeds, livestock, tools, etc that they would need to set up as farmers on their own.
In addition, there was the social challenge they faced. The whites who surrounded them had only bad feelings for them. In the face of this and their economic problems, they had to go about creating a free society for themselves (with churches, families, clubs, etc) for the first time.