Ferguson has been a hugely successful manager, but for all his success, he has only managed one Prem team, who just so happened to be the richest Prem club for most of that time, so we won't know if he would have done anything at another club, especially a poor one like Everton. My guess is that Moyes will have success at United given time. He needs to know his players and how to use them. Saf had 26 years of adding to what he already knew, so it's fair to say Moyes will struggle at first, or anybody else.
Let's see how Moyes and indeed Bobby are doing once the 25 players are of their choosing, as at the moment, Martinez is using Moyes' players, and Moyes is using Fergies.