While by no means the first choice for critical factual detail on a preaching biography, saints lives often contain reliable core facts on the lives of important leaders of the catholic and orthodox traditions.
Many great orators of the church were later canonized, including St. John Chrysostom, St. Bernard of Clairvaux, and St. Francis of Assissi.
Where factual data is shaky, these sources are still a great resource for legend and lore.