Todd Haley was a disaster as HC and, for some reason, gets WAAAYYYY too much credit for his time is Arizona. The Cards were running Whiz's offense, had their best OL with a few years of continuity, had Fitz, Boldin, and a healthy Breaston(his impact often over-looked), and Kurt Warner on the offensive side of the ball. It would be pretty hard for that offense to be ineffective.
Plus, some guys are just better coordinators and not HC material. Wade Phillips,Charlie Weiss, Norv Turner, Romeo Crennel, Spagnolo, and Mike Mularkey(so far) come to mind. Haley is probably in that group as well.
Like Johnh said, the players determine the coaches success, especially at the coordinator level. I don't see NE's offense struggling since Weiss, the guy who went to Penn St., or McDaniels left. McDaniels is back and now he's a good coach again.
As for Chip Kelly, he could turn out to be a good NFL HC, but history suggests otherwise. Wasn't Steve Spuurier supposed to revolutionize the NFL, offensively, as well?