The "Chronicle of the Priest of Duklja," also known as the "Ljetopis Popa Dukljanina," is considered an unreliable historical source for several reasons:

1. Authorship and Date of Composition: The chronicle's authorship is uncertain, and it is believed to have been written in the 12th century, but it claims to cover events from much earlier periods. The lack of clarity about who wrote it and when it was actually composed raises questions about its authenticity and accuracy.

2. Historical Inaccuracies: The chronicle contains numerous historical inaccuracies and anachronisms. It mixes historical facts with legends and myths, making it difficult to distinguish between what might be historically accurate and what is fictional or exaggerated.

3. Lack of Corroborating Sources: Many of the events and figures mentioned in the chronicle are not corroborated by other contemporary sources. This lack of external validation makes historians skeptical about its reliability as a historical document.

4. Political and Religious Bias: The chronicle is thought to have been written with a specific political or religious agenda, which may have influenced the way events were recorded or interpreted. Such biases can lead to distortions of historical facts.

5. Textual Variations: There are different versions of the chronicle, which contain variations in the text. These discrepancies further complicate efforts to determine the original content and its historical accuracy.

Due to these factors, while the "Chronicle of the Priest of Duklja" is of interest to historians and scholars studying the medieval Balkans, it is approached with caution and is often supplemented with other sources to construct a more reliable historical narrative.