The problem facing these tools is a catch-22 really - diagrams are useful when understanding big messy codebases but big messy codebases are hard to visualise.

For instance, I frequently build small paper diagrams of different code paths through a component and nearly always find leaky abstractions, mixed layers of abstractions, weird cyclical dependencies, etc. etc. and there really is no clear way to diagram this. Instead, you sort of have to make judgements and assumptions to make the diagram concise and understandable; the sort of decisions that machines just aren't that good at.

On the other side, when code is simple and easy to follow then the pay off of building a diagram just isn't there.