That said, it sure seemed easier to get nice skin tones from the D-21 footy than the Red footy in the film out test.
I have dealt with both (I think Blair may be referring to me as one of his sources) and can confirm what Blair is saying. The LogCFilm matrices available from Arri are specifically tuned to yield images that work properly in Cineon log format when viewed through a typical print preview LUT, both in terms of their absolute values and the colorimetry they provide. In other words, while Red is basically targeted towards electronic display, the D21 when processed using LogCFilm matrices is targeted towards film output, and the colors and values are appropriate for that format. In my opinion, this is one of the primary reasons that Red has underperformed in "standardized" film targeted DI tests, and Arri has excelled in those same tests. This is not to say that one can do things the other can't, it is, however, to say that each manufacturer has made choices as to where to direct their efforts regarding intended output targets. Perhaps (and hopefully) the upcoming RedFilm transforms will help to address this. Even more hopefully, the Academy IIF proposal will be universally supported and basically eliminate the problem.