Assault is a much more commonly used word. Typically, describing an actual physical attack or military attack. Assailed is not commonly used, except maybe in academic language, history books, or literary language. Assailed, if used in a casual sense, may be used more often in a sarcastic or exaggerative way. Like: "I just came in the door and he assailed me with questions." Or, I would probably say "bombarded" with questions in that example.