I would say they're complementary. Digikam is more of an all-around organizer and has lots of import/export stuff, tagging, and now good face recognition too apparently (the old one was based on classic Haar cascades).
Darktable is for selecting a series of shots, going through them, developing the ones you want to keep.
Darktable's targets digital photography enthusiasts or professionals. The learning curve is steep. Picasa's target demographic was "everyone". So probably Digikam.
I can confirm. Every now and then I want to edit some of my photos and run Darktable, only then to remember it has zero automation and a hundred of sliders for individual adjustments. It definitely requires dedication to use, more of that than I have to give for holiday photo editing.
You can get by using the same 5 sliders for almost all your photos. The other sliders remain available for edge cases or when you want to go the extra mile.
I'm not sure how seamless the experience is on macOS, but I'm happily using digiKam on Linux as an iPhoto/Photos replacement.
It's nowhere near as polished as Photos, but it's capable, open source, and multi-platform. I hope to maintain my photo library for decades, so avoiding proprietary software / service lock-in was my most important consideration. This decision was reinforced earlier this year when we had to do some surgery on my spouse's meticulously curated Photos library: it's damned near impossible to completely preserve metadata when moving items between Photos libraries, and it is impossible to remove RAW files from stacked RAW+JPEG pairs in the Photos library. If I ever ran into those issues in digiKam (which I haven't), I'm confident that my SQL abilities would be sufficient to work around them.
Darktable is for selecting a series of shots, going through them, developing the ones you want to keep.