It's not just trees on Ancestry, it is the same for trees on any genealogy site, on the web open to the general public. Use trees as a guide, contact the owners to see how you are related but at the end of the day use them as you would any suggested record hint that the genealogy sites chuck at you if you have a tree on their site. There are also some very good, well researched trees on sites that show sources etc so it is not all doom and gloom regards them. Sadly the amount of poor, bably researched trees appear to be increasing and this is in part the way people compile them these days and that is a whole different subject. But incorrect trees have always happened on any genealogy site which has a tree building function.