If you can afford to build a robot, you can afford to construct a steel box to do some reasonable testing in.
and energy is only part of the reason, energy of a bullet is reasonably low, but it has such a small cross sectional area that that energy is concentrated.
a spinner despite having more energy is not going to fly as far or deliver as much damage at larger distance compared to a bullet.
Doing the maths off the top of my head IIRC some of those spinners are like 18inch at upto 6000rpm which puts an object (some can be small fixings not unlike a bullet) coming off the edge at around 300m/s or around pistol calibre muzzle velocity sure won't have the penetration of a bullet but not far off - you'd need a good thickness of steel to stop it.