The reason they could have done well at Jets (even though they finished 6-10 and fired) is that they had the right personnel to run their scheme. On the BIlls, they had a defense that was ranked 4th but played exclusively 4-3. They tinkered that defense, sent Kiko to Philadelphia and put Preston Brown instead.
Just like how average players don't adapt well to a new scheme, average coaches have no ability to adapt their schemes to the personnel.