Figurative language, at least in part, is comparing unlike things (often something familiar to something unfamiliar) in order to enhance the author's meaning for his readers. When it works, it is an effective tool which adds to the readers' understanding of the work; when it does not work, readers are left to puzzle out (or muddle through) the meaning.
One danger of using figurative language is that a reader may not know the unfamiliar thing. For example, if a reader does not know that David was a teenage boy who killed the mighty giant Goliath with a simple stone from his slingshot (after many adult soldiers were too intimidated, over a long period of time, to even face the giant), they would not appreciate the comparison of anything else to David fighting Goliath.
A second possible problem of using figurative language in poetry is that it can be misunderstood or misinterpreted. Unfortunately, this happens a lot with poetry, even when there is not a lot of figurative language. Because the nature of a poem is to evoke meaning in just a few words or phrases, the poet uses whatever devices he must to get the most meaning out of the fewest words. Figurative language allows him to do that, but the risk is that he might be misunderstood. For example, if a person is compared to a bouncing ball (resilient, able to rebound from adversity), a reader might pick the wrong qualities for comparison (full of air, very round). This is a rather obvious example, I know, but the principle is the same for more complex metaphors and similes. In fact, the more complex the comparison, the more likely it is to be misunderstood or misinterpreted.
Finally, a reader of poetry may not have the skills to distinguish figurative language from literal language. "A Dream Deferred, by Langston Hughes, is full of similes, any of which could be taken literally by some readers.
What happens to a dream deferred?
Does it dry up
like a raisin in the sun?
Or fester like a sore--
And then run?
Does it stink like rotten meat?
Or crust and sugar over--
like a syrupy sweet?
Maybe it just sags
like a heavy load.
Or does it explode?
All of these images are familiar enough that, for some readers, they are literal rather than figurative.
Figurative language is always a risk, even in conversation. We can make a faulty (or insulting) comparison, we can be misunderstood, we can cause confusion, and we can be accused of not being direct enough. On the other hand, when figurative language is used effectively and is correctly understood and interpreted, poetry is even more beautiful and captivating to the soul.