The draw to him is that he is a good recruiter with great ties to the DC/Maryland/Virginia area. He is also very well liked by many, maybe all, of the players. He is said to be a team unifier (that may be because he was the position coach of the most "upset" players this year - the WRs).
The source of most of the cons, mine included, is his OC record, and that he was an RPO guy, at least at Ill.
Years as OC and the total offense ranking:
Ill: '05: 70-119
Ill: '06: 69-119
Ill: '07: 33-120
Ill: '08: 18-120
Mar: '12: 121-124
Mar: '13: 79-125
Mar: '14: 114-128
Mar: '15: 88-128
In summary 2 offense in the top half of the country in 8 years. Those 2 years he had a great QB in Juice Williams, who along with RPOs, was a revelation to the Big10. They were unprepared for him and it. OTOH, Ron Zook was the coach and Illinois was the program. But Zook did bring in a bunch of talent. ML did have 2 good offenses.
Maryland was the Randy Edsall years, with ML being the OC the last 4: 4-8, 7-6, 7-5, as interim HC 1-5, total year 3-9.
Again, if CNS thinks he's the best hire - OK. And I doubt he would hire him based primarily on the players desire. Play calling is a unique talent, some have it, most don't. Of course, a coordinators job is more than play calling. But that aspect is a talent.