Why is the reliability of church historian Eusebius' The Life of Constantine doubted and how does the tone of the work affect that doubt?

Eusebius of Caesarea (260/265 – 339/340 AD) is sometimes called "the father of church history". In addition to his Life of Constantine, he also wrote the Ecclesiastical History,which is the basis for much of our understanding of the development of Christianity leading up to the Council of Nicaea, and several other works of church history, biblical criticism, and theology. He is generally admired for his industriousness in consulting original documents and eyewitnesses where...

Eusebius of Caesarea (260/265 – 339/340 AD) is sometimes called "the father of church history". In addition to his Life of Constantine, he also wrote the Ecclesiastical History, which is the basis for much of our understanding of the development of Christianity leading up to the Council of Nicaea, and several other works of church history, biblical criticism, and theology. He is generally admired for his industriousness in consulting original documents and eyewitnesses where possible, citing sources, and attempting to be accurate and comprehensive. In his Life of Constantine, he was also an eyewitness to some of the events described and personally acquainted with many of the people he mentions. Nonetheless, this work has certain elements which make it unreliable by the standards of modern scholarship.

The first element is tone, which is that of a panegyric. As with much of ancient biography, the point of this Life was not that of pure factual accuracy but of telling the story of a moral exemplar to praise how a king ought to behave. Contributing to this adulatory tone is that the Life is presented as showing how Constantine fulfilled a divine plan for establishing a monarch as an earthly tool of his divine will, a philosophy that came to be known in the Greek east as "Caesaropapism". Eusebius omits many episodes that are unflattering to Constantine and glosses over others to cast Constantine in a favorable light. He misrepresents Licinius as worse than he actually was to justify Constantine's feud with him. Eusebius also at times inflates his own importance and exaggerates his level of personal contact with Constantine.