By bringing in a supposed good manager who would fail, he makes himself look even better?
There have been multiple recommendations for clubs to get managers from guys like Fergie before, who have been turd managers who have done poorly. Old boys club, would Fergie do that to Utd rather than other clubs, doubtful but the simple fact is a great manager at one club can be crap somewhere else, and vice versa. If the managers style of football, communication, training, rules all gel completely, or are complete opposites to what the players want... you can have a completely different outcome.
I do think Moyes poor record against better teams is, a bad sign. Ultimately Everton might not have had the squad to win cups but cup like games, big games come around every season and they lost a hell of a lot of them, even when basically minnows can come along and spank utd now and then, Derby did IIRC even when they were freaking awful. It doesn't speak brilliantly for his ability to get the tactics and motivation perfect for one game every now and then.... with Utd really aiming for cups and trophies every year it will be interesting to see if he can get the important games right when they come along.