I can only speak for myself but I find using a smartphone upright one-handed for longer than a few minutes uncomfortable because of the bizarre positions it forces my fingers and palms into. It's a flat, slate-like object and our hands are designed to grip round things that protrude into our palms. I also need to keep my thumb free to use the touchscreen and my hands are on the smaller side so I end up balancing it on a protruding pinky and contorting the inside back edge of my hand. It's no problem for me to use two hands or hold it in a screen-up orientation but holding it in front of me for more than a minute isn't fun (despite having decent dexterity and grip strength from deadlifting, typing, playing instruments, etc).