I live in Boise, so I already knew of Mike and the success he had here as an Offensive Coordinator. I also went to ND with Mike's sister and his BIL.
Also remember that Mike Sanford worked with Jim Harbaugh at Stanford prior to his departure and Shaw's promotion. I really liked what he did with the running game at Stanford and Boise State during his time with those teams, and I hoped that would translate to more success with the running game at ND. Unfortunately, I think we saw that Sanford wasn't truly given authority, responsibility, and autonomy to run his offense, and Kelly's Offense by Committee did not produce the desired results.
To answer your questions, I think that given the constraints placed on him by Kelly, he was neither a success nor a failure at ND. He wasn't allowed to do what he was hired to do, and I'm not sure why he took the job other than to be connected to ND for future opportunities (at ND or elsewhere). I think he was at least equally qualified to the OC at ND in comparison to Long, and possibly slightly more so because of the caliber of teams and coaches he had worked with previously. I am not worried that he is gone; I think our offensive success depends on whether Kelly can truly relinquish play calling to a competent OC, and I think it remains to be seen whether both of those components will ever happen. Finally, yes, I did think Sanford was going to help the running game if Kelly allowed him to do so.