"Often, you can find out heaps of info from a public tree."
Well yes and no. Sadly, a vast number of public trees are little more than the end product of a game of Chinese Whispers. In other words, one person somewhere has decided that Person A is the son of Person B and has uploaded this information into their tree with no source citation. This then gets taken as gospel by others who simply copy the "fact" and place it on their own trees.
Public trees should be taken with a massive pinch of salt - they are not reliable sources, only potential clues.