I’v drawn a lot of diagrams, it’s my experience that it’s always about scope and purpose which determine the level of detail.
Scope in how it fits between existing diagrams, like for instance high level CakePHP at a Glance - 4.x and Middleware - 3.10 (even if these are not UML, how do these diagrams relate? Maybe some extra (UML)-diagrams needed, maybe it’s ok now?
Purpose in who’s the reader and what is s(he) looking for? Manager, NOOB, core-developer, debugger, student, programmer? All of them will need another level of detail.
And in the end? 99% of the diagrams were only used by myself to organize my thoughts, to get things clear to myself. Just to be able to discuss things with managers, analists, developers.