There can be several types of frustration.

One type stems from the therapist own needs to be useful and when he does not feel that he is useful and accomplishing anything then he might feel frustrated. I would basically call it counter-transference and if not kept under check can be harmful to the therapy.

Another type of frustration can be symptomatic, indicating that the patient is ready to take a next step but is not doing that for some reason. This can be also called counter-transference but is basically something that comes from the patient and that can be used to further the therapy.

My own T has been few times frustrated with me because he has felt that I am ready for doing something new or more. In this case expressing the frustration in a genuine way can actually be quite helpful.