Clearly, Homer's two very famous epic poems cannot be purely factual. At least, we have no reason to believe that there once existed creatures such as the cyclopes or Scylla. Therefore, these are clearly not completely factual.
But we do not know that the two epics are completely fictional either. For example, we do know that there was a war around a city that is most likely the Troy that Homer talks about. But we do not really know that the city is Troy and we have no clue if the war really happened the way the Iliad says it did (although the city was destroyed).
So some parts of these epics are clearly mythical. As to the more realistic stuff (like the Trojan War), the link says
Whether or not this war really occurred, or occurred as the Greeks narrate it, is a relatively unanswerable question.