1. Your readers can know only what your protagonist knows. You can’t have any scenes in which the central character isn't involved. (A few writers, such as James Patterson have developed a first-person/third-person style. Alex Cross speaks in first person; the antagonist, in alternating chapters, appears in third person.)
2. In older works, such as The Razor's Edge, W. Somerset Maugham himself acts as narrator and tells the story of Larry Darrell and his spiritual journey. That approach has gone out of style. Today, your lead character tells the story. You stay totally inside that head all the way through.
3. One drawback is the awkwardness when the protagonist speaks of himself or herself. In third-person POV, the lead can be objective, but it's difficult to pull that off in first person.
4. Another weakness involves showing the inner workings of characters other than the hero. The narrator can’t delve into the minds of others or show what others think or feel.
5. The biggest weakness I see is that readers see all characters and events through the eyes of the protagonist, which means that even though the person may be perceptive, the other characters are superficial.
Think carefully before you use the first-person POV.
Is it the best way to tell your story?