There's an easy fix for part of the problem: bind shift-space to underscore. Yes, you still require shift, but IMO it's the stretch not the shift that makes typing an underscore jarring.
Unless you use Emacs (as the author seems to), in which case you have S-Space bound to starting/ending selection. But in general yes, one can rebind underscore to something that doesn't require pressing Shift.
It moves your pinky, your smallest and least mobile finger, to move up two rows, which requires a typical person to completely flatten that finger. Either that, or you end shifting your whole hand. Which means that you end up moving your hand a lot while typing because the underscore may easily be the most common keyboard character encountered in some programs.
Obviously everyone has different shaped hands - but for me, the pinky doesn't need to be entirely flat to reach the underscore. I also don't have to shift my whole wrist.
Either way, reaching the + key on QWERTY-102 is objectively more difficult than the underscore [uses the pinky, is farther away than the underscore from homerow], and also requires a shift-modifier. I use + far more than _ while I'm coding. (And I'm currently writing most of my code in Go, so I use _ quite a bit since it's a blank identifier.)
Maybe I just have big hands, but it honestly doesn't impede my typing at all to add an underscore to the text.
