There were literally dozens of trees on FamilySearch and dozens more on Ancestry who had my great-grandfather married to two women at the same time and I spent years trying to explain that he wasn't. Someone, somewhere in time, had attached the mother-in-law of one of my grand-uncles to his father as a spouse instead of to the father of his wife. This probably happened in personal genealogy software and then was uploaded via GEDCOM and BAM! it is reproduced as gospel ALL OVER the internet. IT IS WRONG!! Does anyone besides myself care that it is wrong? It seems most people don't care enough to correct it. I have given up trying at this point.
So, to answer your question, they may not all be wrong, but a great many of them are, and if you are going to rely on what someone else has posted online as your only "source", your tree will likely be wrong as well.
If all one cares about is extending the branches without regard to accuracy, then they can probably get back to Adam and Eve. If they care about making sure the branches are verified, they will eventually come to the end of the road because there simply will not be any documents available to support taking them further.
I have looked at the trees on Ancestry, including the one you mention, there are no sources on any of them that indicate where Abimelec was born, when he was born, who he married, or who his parents were. I'm sorry, but the proof just isn't there.