Honestly, I think that the effectiveness of any figure of speech depends on the audience it's presented to. For example, when Simon and Garfunkel sing the lyrics, "I am a rock, I am an island," most of us know he is speaking metaphorically, but the little kids in the audience who haven't learned about metaphors yet are most likely picturing something like this:


(just to be clear, a rock and an island.)
And then of course, there are the realists.

Nevertheless, figures of speech CAN be effective if they are relevant to the point you are making.
-"Just like a tree that's planted by the water, I shall not be moved."
Here, we know that the speaker is clearly not a tree, but because they have presented this simile with a situation where the point can clearly be seen, the writer would be hard-pressed to confuse the reader by using this phrase.
I really wish I could come up with more, but I think I've made my point. I leave you with a cliched figure of speech in image form that should give you an idea of what this kind of writer's block feels like.