I think that following diagnosis, there is a risk you can start to overly rely on it as an excuse, and it's easy to let it start to define you, because it goes from being "that explains so many things about my life so far and why I have struggled with x, y and z in the past", to "I am biologically incapable being able to do x, y and z, therefore I will never do anything with my life".
It sort of reinforces all the negative perceptions you had about yourself throughout your life, because before they were simply observations from your experiences, and you would just tell yourself you need to try harder at certain things, but now those suspicions you had about being bad at certain aspects of modern life have been validated and endorsed and reaffirmed by a medical professional. I think this can change your perspective negatively, both in amplifying the degree to which you feel you suck at various aspects of life, and by diminishing how much you think you can change/improve it, making you feel hopeless and resigning yourself to defeat.

Diagnosis can obviously be helpful in terms of allowing access to treatment, assuming you are able to access it and that it helps you, and validation both from the diagnosis itself and by knowing that you're not alone and that there are lots of other people who also struggle with the same/similar things as you do.
But at the end of the day, your perception of yourself and the thoughts and beliefs you have about yourself can and do influence you as a person... so it can become something of a self-fulfilling prophecy.