Custom keyboards often seem to follow different rules when it comes to ergonomics and intuitive keybindings. I have my arrow keys on Fn+h/j/k/l and they're much faster and more comfortable to reach than on any standard keyboard. I have my number row behind the Fn key and yet I'm faster and more accurate at entering numbers because the physical keys are placed better. It's very hard to predict how well a key combination works.

Also, tiny keyboards are usually programmable, so the key labels might not be correct. If your layout is custom enough it becomes very hard to find keycaps which match the setup(and basically impossible if you don't want to wait >6 months).