They do shit almost 50% of the time. The biggest problem of filmic is it uses a logarithm, which goes to - infinity when your RGB values are close to 0. But 0 is a valid RGB value, and Rawspeed is scaling RGB values between 0 and 1, by offsetting the RGB values with a detection of the dark current on the non-exposed pixels, at the periphery of the sensor.
The logarithm is very desirable for what we do, since it aggressively pushes low lights. The problem is you have to clip the dynamic range somewhere, because an infinite DR is not an option. The question is “where to clip ?”, and “based on what ?”.
So you could naively think that clipping to the camera sensor DR is a good start. But the RGB code values have been massaged already, scaled and offset, possibly more than once, when they enter filmic. Because we are near the end of the pixelpipe.
Filmic OCIO does it the other way around. It’s a collection of pre-computed LUTs, so the filmic curve is hard-coded in different variants (more or less dynamic range × more or less contrasted intent), then your job is to adjust the exposure and black level before filmic because it wont change.
Anyway, there is a style with most of the useful sweeteners tuned by me, apply it, adjust exposure centile and black so everything fits in inside filmic’s DR, and that’s the most automated image processing I can offer you so far : Auto-filmic - Aurélien Pierre.dtstyle (2,0 Ko) . The next step is simply to hire me to shoot your pictures…