Caution and a chance to double check that the client isn't going to freak out and back off of transition, if there are doubts of any kind (and there often can be) about the dysphoria.
Or with very young transfolk, simply trying to prevent the onset of puberty, while treatment and talk therapy continues before making a choice that will require surgery, should the client regret growing breasts and becoming permanently infertile.
Infertility is probably one of the major concerns as well with those transitioning at later ages, unless the client already has a family and would be unlikely to want to parent any more kids at their present age. Malpractice charges that involve claims of reproductive harm can generate very heavy damage claims, since calculations can be based on the speculative life long value of not just the plaintiff's lifetime, but those of any children he might have generated, plus pain and suffering of not having such offspring.