The answer is the same for both advantages and disadvantages. To gain knowledge is a double-edged sword for Douglass. While education is a source of empowerment for him, it also gives him a sense of futility. Slavery limits his ability to use his knowledge-slave owners sought to keep their slaves uneducated. Douglass had to educate himself using trickery and stealth, while not letting his owners become aware that he was capable of knowing his plight.
What are the benefits and disadvantages that come with education?
In your follow up comment, you've identified one of the major disadvantages education carried for Douglass, perhaps the greatest: it made him aware of his own mind, and made him aware of the greater world beyond slavery, and just how much constrained he was and how much was being wasted. He has a mind that can grasp learned arguments from a range of cultures, and he's being ordered around and beaten. He can understand freedom, but he's being used like an animal.
The advantages, of course, are many. He learns the logical and scriptural arguments against slavery. He masters language, and in doing so, shows he does not deserve to be a slave. Every word written in his autobiography is testimony to how inappropriate slavery is, and to the way people misrepresent the enslaved as inferior. While a slave, education increases his value; once free, education allows him to fight for the freedom of others.