The problem is, if people aren't meticulous researchers (and most people aren't), their trees are going to have hundreds, if not thousands of people in them who aren't really their relatives, especially if they copy from other people's trees. The chances of something like that being accurate is slim to none.
One of the major complaints about trees on sites like Ancestry.com, FamilySearch.org, etc. is the inaccuracy of the trees that members have done.
I’m active in WikiTree, and it’s constantly being pruned to get out inaccuracies. I haven’t followed how the DNA is affecting the tree structure yet. And even when people TRY to get it right, the books they’re working from are inaccurate. It’s hellish trying to straighten the trees out. Often impossible. Just hope that the data is in a form that someday they’ll be able to manipulate more effectively. LDS often seems to just union together everyone ever listed for a couple. I tend to avoid links that don’t have exact birth dates.