Depends what it's a sphere OF. A sphere of mass in outer space (i.e., relatively far from other masses) held together by its own gravity can be very stable - it may remain spherical or almost so for millenia. A sphere of water on a table in my kitchen, not so much.
Also depends what kind of stability you're talking about. If you mean, e.g., a hard ball lying on the flat surface -- it's relatively hard to crush (harder than a cube of the same material), but very very easy to roll (unlike a cube of the same material).
The sphere as a metaphore is meant to be discussed in use in the body. No one really cares about wandering off into the weeds with more mechanical models that appear here that help no one-including those who draw them and have nothing unusual to show for there "understanding."
As is often discussed, go meet people who have demonstrably unusual skills...and more importantly have students..WITH UNUSUAL SKILLS???? If not why
Everyone else? They obviously either don't get it, or have it and won't teach it, or don't want to know. They are the Tom, Dick or Harry who make up the bulk of the martial arts. If you want to be Tom, Dick, or the average Jane...go for it. But is that really what we want for ourselves? Who we want to be?