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There are pros and cons on this possible signing as with every other candidates.
The most important thing is that with McCarthy you know what you get. A very competitive coach, who has not been able to take the next step after winning one Super Bowl.
To me, his weakest point is to keep calm in high pressure situations. There was terrible management of plays and clock on some of Dallas' playoff games. Makes you wonder about preparation and communication on the last 2 minutes of the game.
One thing I will be very curious about, is McCarthy building a staff. In Dallas, when named coach he inherited Kellen Moore and hired Dan Quinn. It seems that Mike Zimmer was another Jerry Jones' hires.
I think McCarthy will be comfortable inheriting Kubiak and Rizzi, if no better choices are available, but I have no clue how will he build a new defence for the saints.
I'm not sure that McCarthy and Kubiak work on the same staff. I believe that McCarthy is pure West Coast guy and Kubiak is a Shanahan/Kubiak system guy. Maybe McCarthy would like to implement that system, but it's not a natural fit.
As far as Rizzi, it's really hard to have the guy that was just the interim HC on your staff when you are the new HC. Too many guys will be tempted to look to Rizzi if they don't like something the new HC does.
But, McCarthy has been in the NFL for a long time and knows a lot of people so I would think he would be able to assemble a good staff and probably already has a group that will go with him wherever he goes.