Back in the day there was an argument where some thought Giggs could walk in and others, myself included, wanted him to prove himself elsewhere first. Then I think it was only Pep that had succeeded in his first job then Zidane did it and Ole, to a lesser extent, almost pulled it off.
I now believe that in some instances, not all, former players who understand the internal dynamics and who possess the coaching talent, mettle and get support from their club can succeed. The key issue is they would have access to more resources but through knowing the club they are one step ahead and those two factors could neutralize their drawbacks.
Carrick is talented, pragmatic and knows the club. With a proper recruitment structure behind him you don't know what he could achieve. Previous success elsewhere is no guarantee of success, we have seen that here. I think there intangible nuances that we just don't see because they are based on the character of an individual, for example ETH is a good coach but his stubbornness/arrogance and naivety could be costing him this season.
CL, PL and other major league wins are fine but I am more concerned with style, tactical adaptability, in-game and man management because of you build a squad strong and deep enough any manager with the above can compete.