When the client consents to ABA and it respects their boundaries, it goes well. The BCBA Code of Ethics even has incorporated this stance, calling it Assent ABA. For the individuals that are treated for Self-Injurious Stimming, such as head banging, ABA without assent has helped reduce the long term damage that these behaviors cause and improve overall mental health, even when it was with a nonconsensual participant (caregiver-only approval).
When the provider emphasizes rewards and consequences, masking and shaming, I have seen teens/ adults give up on interacting with peers and society due to mistrust. Autism is already an isolating disorder, non assent ABA has made it worse for a large portion of the adult autistic population.
In my practice, we emphasize Collaborative Problem Solving instead. It has the same level of Evidence (3: promising) as ABA and prioritizes informed consent. Here are their ratings from the California Evidence Based Clearinghouse (CEBC):