The online trees I tend to use the most for research “suggestions” are ones that contain sources–that cuts down on the number quite a bit if you also eliminate those that only cite other online trees. Sourced trees may reference items that I have overlooked in my research or that I simply have not yet had the time to look for.
Online trees can be full of errors–sometimes. There are times where the compiler has merged people with similar names into one person or has taken several people with similar names and melded them into two individuals.
Other times the compiler has pulled one record from beyond left field to be included in a person’s file when the rest of it is spot on. And occasionally the compilation is entirely correct. Don’t just copy the information from the trees into your own–even if it appears to be correct most of the time. Use the tree for clues.
If you decide to use an online tree for clues, pick apart each specific statement it makes (about a birth, a death, a residence in a location, a marriage, etc.). Look at each record entry individually and ask yourself, is this for the same person? Could this one entry be for someone else?
Also make certain that the record has been transcribed correctly and that there are not clues in it that have been overlooked.