The main one i would add is a lack of symmetrical alignment. the point of a diagram is to create a shared abstraction for reasoning about a system or set of problems. The point of that is to scale work on it to other minds. It should enable others to parse it and ask useful questions.

If your diagram is ugly, you're probably mixing levels of abstraction without acknowledging it. It's a forcing function on articulating what you know and what is outstanding. Something that is black boxed should be referenced as a black box.

I use a lot of data viz because it's a high bandwidth way to show relationships, dynamics, order of complexity and its location, information problems, scope, and de-noise data. So much can be explained by having AI make you a uml sequence diagram of a concept. it is unreasonbly effective. If you are making a "chart for management" and using powerpoint or native excel charts, you're probably creating garbage though.