A progress report; I've come as far as I'm rewriting the docs affected by the new behavior. Usually one discovers issues when testing the workflows though so it may still be a while.
Gamut compression which now happens in several places seems to work well, and the new weighting and target adjustment functionality gives better control of the colorimetric base profile result than before.
Tone reproduction look remains largely the same, with a liiiittle lighter blues per default (the new weighting allows making Adobe/C1 style light blues if one likes that, but being plain wrong per default is too much even for me
). Clipping behavior of bright colors (with in-gamut chromaticity) has changed to the better.
I'm still working on a very difficult problem though. I've got an excellent test file which is a sunset in haze, producing a loooong smoooth gradient from reds to yellows and into clipping at the sun center. ACR profiles renders this quite well although they "cheat" a bit by letting many neighboring colors become yellow and lighten them a fair bit which hides gradient issues. The file does look like a "fried egg" without a curve like sunsets do so a 100% perfect rendering with no visible bands in the gradient is not possible, and ACR doesn't do it either but it's as good as it can be.
The bands are not "posterizing", but simply that one channel starts changing a fair bit slower, so there's mathematically nothing wrong with the gradient, but the eye sees the flattening as a band. These bands often occur as well when one channel reaches clipping and the other continues to grow. You can't solve these issues 100% with a profile, but the profile can reduce or increase the effects.
With DCamProf I have quite clearly the "fried egg" issue with sunsets with such bands.
The source of the problem seems to be that models of luminance are not precise enough when translated to RGB. As soon as you do anything with some other color model than RGB (like JCh, IPT etc), gradient issues are likely to occur especially when you have a transition in hue at the same time as the luminance increases, like in a sunset. The eye is extremely sensitive when it comes to detect gradient issues, sub DE 1 for sure. I'm looking at how to improve the situation, without falling back to an RGB-oriented model like I think both Adobe and C1 has. An RGB-oriented model is better at handling extreme ranges, clipping and gradients, but it's worse when it comes to the general look of colors.
DCamProf is already today a hybrid, using more RGB models in the extreme ranges and clipping, but it still doesn't work as well as I'd like.
Evalutation is a bit tricky though as it's affected a bit by the screen, color management can mess up gradients too.