I think therapists can do plenty of damage in the here and now, independent of the client's early childhood problems. If your present reality includes e.g. isolation or illness or recent loss, and you get mixed up in a dysfunctional relationship with a therapist, could by very stressful. The usual framing is that client distress is due to their "attachment style", and then the therapist is off the hook as usual.

As for adults with attachment disorders and so on, I think a major fallacy is this idea that therapists can be trained to work with such people. What does "work with" even mean? Usually it's implied to mean having a relationship wherein the client "attaches" to the therapist, learns how to have a secure attachment, then re-enters the world fixed. This ain't exactly scientific, nor even logic-based. It's more like a religious conversion. My guess is that this process is nearly always improvised, and thus is not really a "process" at all, but a psychological experiment, carried out on the backs of clients.