For historians, oral traditions are good because they give us insights into the ways in which people perceive their pasts. On the other hand, they are bad because the perceptions that go into the oral traditions are not necessarily accurate.
Historians want to have sources that tell them about the past. In some societies, written evidence is lacking. In some parts of society, there are oral traditions that are not the same as the traditions from the official records. Historians find these sources valuable because they give us ways of understanding the past that would be unavailable otherwise. If, for example, we have no objective written records of Native American history, oral traditions can tell us things about how they lived and how they perceived their history.
However, the problem is that these oral traditions are not necessarily accurate. Traditions can change as they are passed down from generation to generation. They can be based on how people want to see themselves rather than how things really were. Therefore, historians can get mistaken impressions from these traditions.
Thus, oral traditions can be good because they give us insights into things for which we lack records, but they can be bad because they are not always accurate.