Nothing to show? The dude is the winningest coach at his alma mater. He was offered the Michigan job. He has basically earned a lifetime appointment to a $2+ million per year job. You must have accomplished a helluva lot by 38 years of age if you think that is "nothing to show."
Ferentz had a great run. About a decade ago. When the fingerprints of guys like Bielema and Joe Philbin were on the team. And when the Big Ten was "line up and punch 'em in the mouth." Those days are past. Ferentz put together a monster run in 2008-2009 and we got a lot of bounces to make up for a lot of crap from the years prior to that run. But even in the midst of that monster run, the guy freaking lost to Fitz TWICE. AT HOME BOTH TIMES.
You can talk all you want about the relative talent on both squads when the respective coaches came in, but the fact remains that Ferentz can do some things Fitz can't. Ferentz can sign a guy like Faith Ekakitie who wants to go to NU but has too low of a GPA or ACT score to get in (but who is welcome at Iowa). Ferentz can go to a juco and pick up a guy like Brad Banks, Nathan Chandler or Marshal Yanda and plug a hole immediately with a junior rather than an underclassman (NU does not allow Juco players in unless they could have qualified as freshmen). Ferentz can park a guy like Shonn Greene at Kirkwood for a year until he can regain his eligibility (at NU, if you're gone academically, you're gone). NU actually has an academic brand that is about 1000x more valuable to them than a football team and they go to great lengths to protect it.
You can create your own little narrative about how awesome Ferentz is, and I will readily admit that he is a solid coach (albeit not a near $4 mm per year man), but when you look at the totality of the circumstances, Fitz is about 5x the coach of Ferentz.