Another problem with Ancestry is that a lot of times people will accept Ancestry hints without checking to make sure they are accurate. I've seen trees with people who died in 1847 and then are living happily in 1860. I brought this up on another forum once though and it made me really unpopular.
And another problem I often see is that Ancestry will automatically insert a woman's married name unless you remove it or change it. That can really send someone off on a wild goose chase.
Use Ancestry for record hints, and review them carefully. You can use the family trees as reference, but verify, verify, verify.