One way to deal with criticism and complaints is to not insist on proving your clients that you are right and they are wrong.
It's tempting to challenge criticism, to explain your procedures, to justify the choices. When you do that, it makes it more difficult for the client to share his truth, to feel heard, and the back and forth it generates inevitably escalates the tension.
Instead... Accept the feedback. Don't judge. Sit on the same side of the table and search the best path forward.