Harriet Jacobs was an escaped slave who eventually became an abolitionist and wrote her autobiography, Incidents in the Life of a Slave Girl. As your question suggests, Jacobs has some hesitations about the book; however, her primary hesitation was whether to publish it, not whether to write it. Her fears about publishing this work are valid.
First of all, she risked being seen as a liar because most slaves could not read or write. Jacobs learned to read and write when she was young ( a fact for which she is grateful since she was able to read the Bible in many of her most difficult times), but many people would not have believed that she could write, and certainly not well enough to write a book of this quality. In fact she used a pseudonym for herself, and for many years the book was not ascribed to her, though Jacobs is now recognized as the author.
Second, in telling her own story she necessarily revealed other people's stories. Though she used pseudonyms to disguise specific people, she understood that her friends and family still enslaved could suffer the negative effects of this book. Though she did not deliberately portray the people in her life as fiends or monsters, a simple retelling of their actions would certainly cause others to see them this way. Also, she was an escaped slave, which means she could have compromised the underground railroad system, so she had to be protective of those people and systems, as well. It was a risk that she ultimately decided to take, but she did hesitate because of it.
Finally, she is honest about her sexual abuse at the hands of at least one man in her life, and because she did not extricate herself from his control, she understood that she would not necessarily be seen as the victim she was. We have a much better understanding and a heightened sympathy for her situation today; in her own time, such revelations had the potential to make her an object of scorn.
The woman who edited Jacobs's book, L. Maria Child encapsulates the conditions which, at least in part, caused Jacobs to hesitate in this quote from the introduction of the book:
I am well aware that many will accuse me of indecorum for presenting these pages to the public; for the experiences of this intelligent and much-injured woman belong to a class which some call delicate subjects, and others indelicate. This peculiar phase of Slavery has generally been kept veiled; but the public ought to be made acquainted with its monstrous features, and I willingly take the responsibility of presenting them with the veil withdrawn.
The subject matter--slavery, sexual abuse, sacrifice, escape--would be seen as "delicate" by some and "indelicate" by others (too sensitive for some to read, too vulgar for others). The subject of slavery, especially in 1861 when Incidents was published, was rather secretive and shameful. People either understood it and did not want to discuss it or did not know much about it and wanted to keep it that way. Those who decided to publish it, including Jacobs herself, believed this was one way in which the "monstrous features" of slavery could be exposed and eventually dealt with.