How successful is fertility preservation?

As with fertility treatments in cisgender people, there is no guarantee of success. Not all sperm and eggs may survive the defrosting process, and there is no guarantee of successful fertilisation. However, there is no difference in success rate between frozen sperm/eggs and fresh sperm/eggs.
If you are considering fertility preservation ahead of medical transition, it is best to do it before you start taking hormones. Although there is promising evidence that fertility is possible after coming off cross-sex hormones for both transmasculine and transfeminine people, there is no guarantee that sperm count/egg production will return to pre-hormone levels or resume at all.