The resemblance should neither be an empty cliché nor so remote that the listener has no route to understanding it. The beauty of metaphor often lies in balancing intelligibility with imaginative force.
A distant resemblance can stimulate thought in an explicit simile. Metaphor, however, goes further by speaking of the subject through the borrowed image itself. The most useful novelty is one that advances understanding rather than burying it.
Rhetorical excellence depends on audience and occasion. A connection obvious to one circle can require explanation for another, so the context of communication matters.