> Here's a challenge, to help appreciate the nature of these: try to find an error in the diagrams.

The point of diagrams is to communicate concepts and ideas. They are a design tool, not a design goal. They are used to help reason about your design. One of the most basic uses is to step through each of your requirements and verify if the design meets each and every single one of them.

> For example, large amounts of work that have to be redone, or a project abandoned (...)

No, this is not the diagram's fault. That's a problem caused by poor or even inexistent design. You only need to redo a project if you fail to meet critical requirements and you can't figure out a way to make them work with what you have. The diagrams only convey the output of your design process, and if it is faulty then it's not the diagram's fault.