First of all, I'm not opining whether he would be a good choice or not. The fact that his name "came up" may be a reflection of him trying to create a market for himself rather than truly being interested in either ND or college coaching. But the problems are obvious. It is odd that he is trolling for a job on college when you would think there would be a demand for him in the pros.
Leftwich has never coached in college. Hence, unlike another "fantasy" candidate Joe Brady, he has never had to recruit. And the new OC has to be a strong recruiter. We don't know if he would be any good at it.
The other problem with Leftwich is he is always going to be "in the news" every time there is a head coaching job in the NFL which, in the past, he has indicated was his career goal. Until Leftwich "commits" to college coaching, he is always going to hear the calling and anyone he tries to recruit is going to know this because every single schools' recruiting staff is sure as hell going to let them know.
Golden, for example, has coached in both pros and college and his return to college coaching may indicate that is where he wants to stay.