I think it has too much regimentation to be a good communication device, or even a good design specification. Labeled boxes, arrows, and sometimes color are more than sufficient for diagramming how a design works. The small details in a design doc are not going to stay the same when building the thing, so writing them down is just a waste of time.

(Personally, I’m a fan of doing less design, less documentation, and making the code as obvious as possible. It’s a lot of friction to have multiple sources of truth that have to be kept in correspondence with each other.)

UML was, at least partly, advertised as something that would make software developers obsolete. You just needed domain experts to wire up the right class diagrams, sequence diagrams, activity diagrams, etc., and it would output code that met the specifications. No developers needed!

Imagine you want to have a conversation with a colleague about how to approach a new project, but some smart-ass is constantly pestering you, forcing you to use a heavily formalized, technical language that requires you to put every idea into a well defined box. Caveats, questions and loosely defined entities, processes or relationships cannot be expressed.