I don’t think there is any one thing that specifically went wrong in the Fangio/Dolphins relationship.
Is Fangio a very, very good defensive coordinator when all the pieces are in place and things are running the way he likes? Yes.
Is it pretty damn apparent he wanted to be in Philly even while coaching in Miami? Yes.
Was he right about the lack of discipline and work ethic of some of the players on defense? Yes.
Does he seem to be a grumpy, rigid ***hole who alienates people and refuses to adjust when things aren’t working perfectly? Yes.
All of these things can be true. I’m glad that things worked out for him in Philly. There have been plenty of instances in which things didn’t work out with him, and it wasn’t just in Miami.