The Wayback Machine usually only archives HTML and plaintext but often does not copy pictures, movies and other multimedia from the websites that it crawls. Further it doesn't copy entire sites. Usually only the first few pages. When it does copy, it only copies the changes from one time period to the next. Therefore, dynamic websites like web forums, blogs or social media sites can not be copied reliably all of the time.
Wayback is good to let you know that a site existed at a particular point in time but its not always a reliable record.