Online family trees can be a tremendous source of information. They can also be a tremendous source of misinformation.
Unfortunately, far too many trees are totally undocumented or site other undocumented trees as sources.
Others are partially documented, but use the wrong source as documentation. For example, a marriage record may appear as a source for a marriage. That record may have the correct name of, say, the bride, but a different date, location, and different husband. Right name, wrong person.
Some trees may be partially and correctly documented, but lacking documentation for several events.
Then there's the golden tree - totally and correctly documented!
For the imperfect trees, utilize the information, but search for records that support the events. Then you can use the information in your own files. If the undocumented details fit your story, explain why you are including the information and why you are convinced it fits. Then keep looking for sources to support your theory.