They're even more difficult in sign language and a recent study on the use of metaphors in spoken language and various sign languages looks into the issue. The recent paper by Irit Meir of the University of Haifa examines the interrelations between two notions that play an important role in language and communication, iconicity and metaphor.
Meir's study says that the iconicity of a form may constrain the possible metaphorical extensions that the form might take. Put another way, certain metaphorical expressions in spoken language cannot be "translated directly" into sign language if their form is iconic.
Because sign languages were created in modern times with a specific purpose, the are 'natural' languages and often have rich and complex grammatical structures and lexicons. Sign languages have metaphors but often when trying to translate metaphors from a spoken language to a sign language, it is impossible to use the same words.
For example, it is impossible to use the sign FLY (in the American Sign Language and Israeli Sign Language they examined ) in the expression "time flies" or "the day just flew by". The metaphorical uses of a word such as FLY are impossible because of the form of this sign, in particular, its iconicity. The sign for FLY is produced by moving the arms as if flapping one's wings. But in the expression "time flies", we obviously do not mean that time is flapping its wings. Rather, the metaphor is built on an implication of the action of flying, namely that it is a very fast way of motion. So there is a clash between what the form of the sign encodes (wing flapping) and the aspect of meaning on which the metaphor is built (fast movement).
When that kind of clash occurs, the metaphorical use is not possible. The meaning components reflected by the form of the (iconic) verb and the meaning component which serves as the basis for its metaphorical use should be congruent and if they are not, then the sign cannot be used for the specific metaphorical use in question.
Iconic signs, then, are more restricted in the metaphorical extensions they can undergo than non-iconic signs, because their form is not arbitrary. The effects of iconicity on metaphors are much more salient in signed languages, because of their better ability to express many concepts in an iconic way. Sign languages, then, are instrumental in getting better understanding of metaphors and the forces that shape them.
Citation: Irit Meir, 'Iconicity and metaphor: Constraints on metaphorical extension of iconic forms', Language, to be published. Preprint version available here