Great work on the noise reduction 
I find the ‘preserve shadows’ slider very useful. In darktable 2.6 I sometimes used a parametric mask to increase the denoise intensity in darker areas, the new features make that much easier.
This is a photo without denoise:
This is with the default settings in profiled denoise:
And finally with a lower value for shadows preservation:
In my experiences with the new profiled denoise the loss of detail is very low, the darker parts become a bit darker than before and you lose a bit of sharpness in the image. But you can easily compensate that.