If you have only two thumbs available for typing, that can only reach about half the keyboard each (quite a hard limit!), you realize how asumptions this layout makes suddenly don't hold, and normal things like ctrl+a or alt+shift+1, or alt+shift+any arrow are almost impossible.
Yes, you can modify the shortcuts in i3wm,screen,... (not Firefox, which is quite resistant to modifying keyboard shortcuts by design) specially for your "PDA". But if you just want a portable device you can painlessly migrate your tasks to for a few days a year, spending a lot of time designing shortcuts for your DE, or terminal multiplexer, is not that appealing.
It would be probably more useful if key layout was designed from the start to allow various modifier combos to be invoked by dedicated keys, or layout designed so that more modifiers were symetricaly placed (or something even less traditional, and have all letter/number keys on the right and all modifier/modifier combos/special keys on the left), instead of wasting space on big enter key, or big spacebar (for which you may spend several HW iterations just to make those right and reactive on their whole surface), etc. Due to size, it's usually uncomfortable to press 3 key combos even with the keyboard laying on the table.
Also comfortable position for the most comonly used keys (usually modifiers) is not on the edge of the keyboard. That's really pretty bad for your thumbs.
The layout of these things just needs a lot more thought.
Sad thing is, that if someone would spend a ton of research and thinking on making something usable and comfortable, most people would completely poopoo it as a freaky unusable layout, just because it would look unfamiliar, lol.