100% of the functionality without hand movement. You can have the arrow keys directly under your fingertips at all times, same with numbers and special symbols.
And while I don't like this particular layout in the link I do have a very similar 42-key keyboard and after a couple days of practice it was the most natural and smoothest typing experience I've ever had.
Terrible for any one-handed use (e.g. gaming), though, and it's definitely not everyone's cup of tea.
What keyboard / layout do you use to get the arrow keys? I have a Ultimate Hacking Keyboard (https://ultimatehackingkeyboard.com/) and while the arrow keys are easily accessible, they require pressing a modifier.
This is fine and dandy for simple usage, but I happen to do do a bunch of more complicated stuff: ctrl + arrow to go to start/end of line, ctrl + alt for words, all of this + shift for selection. ctrl+shift+mod+arrow gets really tricky to operate for me, I haven't been able to reach a level of proficiency that was anywhere near equivalent to what I had without the modifier key.