A possible link between two subjects is one thing; a recognizable link that serves the audience and purpose is another. If listeners must invent an extremely remote interpretation, the metaphor may be aesthetically weak.
Obscurity of resemblance does not necessarily mean that the figurative intention is logically impossible. It is primarily an issue of rhetorical quality. An appropriate contextual explanation might make a previously obscure image intelligible.
The practical criterion is that the intended sense should be recoverable by the audience through a fitting relation and context, not locked inside the speaker's private associations.