I do not think that any of the programs that I use daily supports radial menus, but the shape of the menus has really no importance, because with the tablet in "Relative" mode and with reasonable values set for the acceleration and sensitivity of the cursor I can reach instantaneously any point on the display, with a very small hand movement, no more than one inch for going from one corner to the diagonally opposite corner of my monitor.

I think that radial menus could have been useful when used with graphic tablets used in their default "Absolute" mode, where the position of the stylus on the tablet corresponds directly to the position of the cursor on the screen, which makes more awkward the cursor movements in locations that are far away from the center of the screen.

No such problems exist when the tablet is in "Relative" mode, when the stylus behaves like a mouse, except that it has neither inertia nor friction and your hand rests in its natural orientation.

Using a graphic tablet in "Absolute" mode is equivalent with using a stylus on a touchscreen, e.g. on a smartphone or tablet, so in those cases using radial menus might also be convenient, unlike in my case, where the shape and position of menus do not matter.