What were the benefits of hereditary rule in the roman empire? what were the drawbacks?

Benefits:

In a hereditary monarchy, all the monarchs come from the same family, and the crown is passed down from one member of the family to another ... When the king or queen of a hereditary monarchy dies or quits the throne (abdicates), the crown is generally passed to one of his or her children, often to the oldest.

Drawbacks:

Most obviously, the ruler is not chosen on merit but by the accident of birth. This may be described as letting God choose the ruler, but as Aelianus has remarked on these pages, He does this through secondary causes and has given us no guarantee that in this case secondary causes, undirected by human intelligence and will, will produce a particularly good result.