Autobiographies are a source. Like with all sources, it's the historian's job to assess the context of the source and the possible private or public interests behind it.

I will expand on this as the answer goes on, but a brief response would be that autobiographies often are one of many sources, and like all sources they need to be cross-checked and scanned for the possibility of "convenient inaccuracies" on the author's side.

Understanding the context of the autobiography already lets you understand it and the points it makes much better. Taking an autobiography at face value without making these considerations can have adverse effects that last for decades.

One particularly straightfoward example is the treatment that was given to the memories and autobiographies of former german officers after the Second World War. When western historiography started gathering and classifying this kind of material, the new conflict that would go on to become the cold war was already taking shape. As a result, there wasn't any real interest in depicting the Soviet Union or their military as particularly advanced or competent.

On top of that, many german officers wrote memoirs and autobiographies detailing their experience in the war with a clear goal in mind: to rehabilitate themselves. Either to keep their current role, to get a new one in organizations like NATO or later the EU, or simply to clean up their future reputation.

In this case, we have a deliberate effort to do things the wrong way from both sides. Generals like Model or Von Bock would often blame their defeat on the eastern front on factors that were natural and outside of anyone's control, like "the winter" or overwhelming numerical superiority. No matter how hard they fought or how competent they were tactically and strategically, there's just nothing anyone can do to win in such terrible weather and against endless swaths of enemies!

This led to a long-lasting and very widespread misperception of the red army and its leadership, effects of which can still be noticed today. For example the myth of the "eastern ordes", endless streams of barely equipped conscripts storming and overwhelming german positions: not only this has led the general public to vastly misunderstand what "human wave attacks" actually are, it also helped build a skewed view of the conflict. By analizing the wider picture, looking at battlefield reports from minor officers and crosschecking sources we can get a much clearer picture, one that shows how the red army outfought, outgunned and outplanned the german army since late 1942.

This is one of many examples of how trusting autobiographical sources can distort the overall perception of the event. Modern historiography posits that history is contextual and complex, characteristics which don't really mix well with the personal and unique perception of a single individual.

Yes, of course, people did lie on their autobiography and many still do nowadays. Some with an agenda in mind, some by mistake because they don't realize the individual premises with which we process events and experiences, some do because they genuinely don't realize or understand that their experience isn't universal. Sometimes the historian itself doesn't understand this:

If i find an account from a soldier in the franco-prussian war complaining about how often his rifle jams, i might be inclined to believe that the rifle models used at the time were very prone to jamming and write that down. This would ignore the experience of hundreds of thousands of other soldiers whom never had such issues. But if i consider this information with the context in mind as an historian is supposed to do, i will conclude that rifles of the time did occasionally jam but without it being a serious impairment for the common soldiery.