CFC supporter here.
I find the inexperience argument baffling. If you look at City squad it's full of seasoned pros, all with years of playing at the top level. Silva, Aguero, Toure brothers, Barry, Nasri etc aren't youngsters, just because they haven't played in the CL for your club doesn't mean they lack experience at this level, they don't. Mancini has been around long enough, too and managed Inter in the CL for years.
Chelsea reached the CL semis first season after Abramovich's arrival and continued to do relatively well for years afterwards. Your owner's investment is more recent and the team is in its prime. Bowing out of CL at the group stage for the club with your depth and quality is simply not good enough, imo.
As for group of death, Villareal were shocking and Napoli is a very good team but not on your level and has no CL experience, either. Finishing second would have been acceptable since Bayern are a top club and were favorites due to their pedigree, but missing out on qualification altogether is a failure, given your resources and Mancini knows it,too.