As a casual photographer, I wanted to love darktable and I'm sure it's extremely capable. But the UI is just so hard to get to grips with. I've put a few hours into it, tried following some tutorials etc. but I have no idea what I'm doing there.
I do have a fairly decent grasp of color science from working in 3d graphics so it's not that I'm lacking there. I guess it's like blender of yore. It could become mainstream but it would require a full UI overhaul and in the meantime it's for experts only, or determined people with a lot more time on their hands than I have.