The criticism for Nua and other fan-submitted candidates has been that he does not have DC experience - specifically in terms of calling plays. Whether you are promoting a position coach from within or taking a position coach from another team, you are still gambling on them be able to elevate. While there is a risk, it is a risk that major football programs take on a yearly basis. For some reason, certain fans like above think we cannot afford a learning curve. This attitude drastically limits the pool of potential coaches - not only in terms of people who have DC experience, but also people who have DC experience that would consider taking the BYU DC position as a step up.
Requiring prior DC experience put us at FCS/G5 coordinators or previously failed DCs.
Taking a high level position coach - someone like Nua, who has experience with major programs (teams with championship aspirations) like USC and Michigan, allows different perspectives and habits (ways high level programs practice, recruit, etc.) to be incorporated into BYU.