It seems that a very meticulous hand is needed to get the corner-shaped pointer to display at the corner of a window in order that the window can be re-sized.
Is there anyway to make this more likely to appear?
Alt + middle-mouse-button click any general region of the window. (Obviously, the only problem here is that you need a mouse with a middle button)
I prefer this workaround to having more accessible re-size zones. That said, I share your frustration. These picky re-size zones are a persistent problem with Ubuntu; I know the problem existed in 12.04 LTS, my first distro. I wish the amazing Ubuntu team would fix it.