From browsing r/ErgoMechKeyboards and GeekHack, I think the main group of people making these custom keyboards are devs actually. Consider how foot pedals are a long-standing emacs thing :)
I use a 40% layout on a custom board with palm keys for coding and writing in LaTeX mainly (like the Model 01 with 3x6 alphas 3 thumbs 1 palm on each side). It took some effort to re-train, but I'll never go back to a keyboard without palm keys now, I love the setup. They palm keys are perfect for activating the layers for arrows/navigation or numpad. But typing style plays a role too, it's definitely probably not everyone!
I use a 40% layout on a custom board with palm keys for coding and writing in LaTeX mainly (like the Model 01 with 3x6 alphas 3 thumbs 1 palm on each side). It took some effort to re-train, but I'll never go back to a keyboard without palm keys now, I love the setup. They palm keys are perfect for activating the layers for arrows/navigation or numpad. But typing style plays a role too, it's definitely probably not everyone!