Bearing balls made me think, a lot of + & - factors.
+ Near zero play; - tiny surface contact. + Easily replaceable, - unsure new ones be same grade. + Many common sizes, - do ball endmills cut real spheres? I'd probably ball cut where a pilot drill removes 20 something percent area of center contact. 'Floating' in high pressure lube and well sized pocket, running good material like shafting might be.
Full bearings are common sure enough, wonder if anyone's run bare rollers from bearings compared to bearing balls?