Hi Levi,
I have one dead and one living alcoholic parent. I have used letter writing with both of them. I consider it to have been a success, but that's because of what I consider to be success.
Writing each of them letters at different points in my journey to overcome my toxic childhood and to build a healthier future for myself and my own son, has been about getting me well not them. I never sent the letters to them. I never expected the letters to change their behavior in any way. I'm the only one I can change and I used the letters to help me sort out my part (usually the victim) in their on-going dramas. The act of writing the letters, like journaling allowed me to be more objective and less judgemental and in many cases to just let go of the hurt.
One aspect of the letter writing that was not particularly "recovered" of me, was that I did not share the letters with them for very specific reasons. I already "knew" what their reaction would be. They tell me in different programs that you can't "know" how another person will react. I maintain that if you live in dysfunction long enough you absolutely can know. And I "knew" that my parents would use the information in the letters against me. First to deny reality of our history and second to choose better buttons to push in the future. I also "knew" that the letters would be very hurtful to my parents who would not use them as opportunities for open dialog and change but as evidence in the court of public opinion as to what an ungrateful, hurtful daughter they had raised.