Is going to exhaust your thumb and cramp the hell out of your hand in record time.

And on top of that, occupy some 'third' control scheme somewhere between "kb/m" and "gamepad" that developers would have to explicitly target, at a time when so few of them even put the time into adding proper kb/m support for their console-first-PC-port-maybe-if-its-cheap-to-farm-out titles.

I find trackballs (the Logitech M570 in particular) much more comfortable to use over long periods of time personally. The only criticism I have of their design is that having the trackball on the side entails removing a lot of area where additional mouse buttons could have been placed, and I enjoy having extra buttons there to bind to debugging/build/vcs keys.