Wyche has been a successul head coach. He seems to get some credit for Esiasin and Montana, but I believe the two of them would have been outstanding regardless. Mcnally only gets the praise locally because he is about the "highest" profile coach from Buffalo. There are much more respected line coaches in the league. Russ Grimm from Pittsburgh as always had a huge national rep at Oline. Dennison has earned the rep as well. Mcnally was overhyped locally well because he was a local boy.
As far as head coach vs. position coaches, I believe that successful head coaches are that way because they have good position coaches, or they can control what they teach. I think this is why so many coordinators fail. Because they are good at one thing or maybe given credit for the headcoaches handling of them. Parcells, Walsh, etc... were outstanding at building head coaching assistants, on the otherhand, Marv didn't do a good job with this and it's obvious from their lack of success in HC spots.