The bigger screen size makes it easy for UI designers to screw up.
For example, my smartphone has a 4.55 inches screen and I can't use fundamentals apps, like making a phone call, without using both hands. My thumb can't reach every part of the screen, unless I hold the phone awkwardly and risk to drop it. I honestly struggle to accept incoming calls and then hang up.
Had the developer of the app decided to use vertical gestures and place critical buttons near one edge of the screen, then I could handle the smartphone as it is intended for trivial tasks.