I don’t have experience with darktable, but moving from Photoshop to Gimp, from InDesign to Scribus, and from Illustrator to Inkscape definitely tool me some time to transition and learn. I’d think that would be the same going from Lightroom to DarkTable. However, once you have invested in the new software, chances that you need to change again ik the future because of shitty expensive subscription models is slim. So I think it is worth putting in the effort.