One thing I notice from pictures of the keyboard.io is that there is empty space next to the Ctrl thumb keys. This seems strange because the missing slot is a premium thumb position, at least for my hands. For example, on a standard keyboard both the Space and Alt/Cmd keys are extremely comfortable to hit; looking at my hand from above, when hitting Space my thumb lies right outside my index finger (corresponding to the Ctrl on keyboard io), and when hitting Cmd my thumb lies between my index and middle fingers (corresponding to the empty space next to Ctrl). Another keyboard I was looking at, the Esrille, also has thumb keys that extend farther inwards rather than outwards - the position of its Fn key right between the index/middle fingers roughly corresponds to the Alt/Cmd on normal keyboards. In general, extending the thumb too much feels like it strains the wrist whereas flexing it slightly is a small and natural motion.
So it seems more natural to shift the 4 thumb keys one space inwards, or just add an extra key in that empty slot. I do know that I have somewhat smaller hands (for reference, can just barely hit octaves on a piano), so maybe it’s just me. But I’ve also seen pictures of people typing on it and it seems hard to hit the outermost key (Shift by default, I think) without moving the whole hand.
I imagine leaving out this particular thumb key was a deliberate design choice, so I’m curious about other people’s opinions and experiences with this.