I know, for me personally, I was unable to effectively address my substance abuse issues until my Bi-Polar disorder was diagnosed and properly treated. It was not a cure for the substance abuse, but with proper medication and CBT therapy I am now able to address and work on the SA portion of things.
One reason I feel that 12-Step recovery was not effective for me was that generally the issues of mental health and substance abuse are generally dealt with separately, if at all. At least that has been my experience.