Thats a very complex answer to a simple question.
There have already been some good answers.
A telling point for me is when somebody makes their own program is whether that program is appropriate for them. They might just have gotten lucky and basically copied one, and it is a good program, but is it appropriate for them?
Does that program address there level of experience, or is it just another copy of Arnolds Olympia program? More often than not they design a program far beyond their current level or needs.
Another point is this. Can they explain the program and why it is designed the way that it is, and how the whole things fits together in the larger picture. It does not matter if it is a split or a full body concept, at the root of it, it is your whole body that is being worked. The overlap areas need to be thought about, and how your hitting things. Progression needs to be a central focus point as well. A entire book could be, and several have been written on this topic. Less is more in writing a good program.
I would venture an estimate, and say that probably 75% of peole who are lifting are using a program far beyond their current needs, or just flat out using the wrong program.
This whole sport has taken a serious wrong turn in the aspect of program design, and what most peole should actually be doing as opposed to what is being currently done. It is really a sad state of affairs that one of the most frequently asked questions by people looking to lift is, "What supps do I need to be taking."