Great article, this quote stuck out for me:

  The same rule applies to visual metaphors: Just as any
  literary metaphor, a visual metaphor confuses if it doesn’t
  clarify; it breaks if you stretch it; and it becomes
  ridiculous if you combine it with a second or third
  metaphor.

  Avoid metaphor that do not clarify; if a metaphor darkens
  the meaning of what you are trying to express, don’t use it.