I might adjust the curve handling in the neutral tone reproduction operator further, I'll do some testing. I'm not 100% sure the 0.9.8 change was 100% good.
A reason I was using a RGB-HSV curve as luminance guide earlier was that the S-dip in the shadow range becomes a bit darker and duller with the pure luminance curve, which had some negative effect on shadowy skin tones. The difference is very small, but it's there. As the curves are typically designed using some sort of RGB curve, it may be wise to have a luminance that better matches RGB. I guess that if you do design the curve with luminance curve in mind you would simple not make as big S-dip in the shadow range and then you'd get rid of the dullness, so it's possibly not the fault of the luminance curve as such.
The RGB(-HSV) curve treats R G and B equal, that is over-estimates the luminance of red and blue. This means that dark colors with low green gets lighter. It also makes specifically saturated reds lighter and tonally tighter spaced, which is why the ACR rendering has worse tone separation in the tulips. The same can to some extent be observed in the blue range, but the perceptual effect is much larger in the red range (as saturated blues are dark anyway).
A pure RGB curve like C1 is using for its film-curve has the same problem with tonal compression of reds, but their ICC profile is compensated for that. Overall ACR seems to "suffer" more of the look side-effects the curve has as their profiles are not as compensated (I think they originally intended that the RGB-HSV curve would be used as-is with no compensations), while C1 has much more designed color.
There's no surprise C1 has better reputation than ACR when it comes to color. It's not an issue of the DNG profile format though, with designed profiles you can get the color you want, DCamProf profiles look the same with ICC as with DCP.
