I've only seen Lars have a spinning riser and I don't think it was going too fast.
You have to take the drummers position on the riser into account. If the drummer is centered on the spinning axis of the riser, the effects of g-force will be different than if the drummer was away from the axis of the riser. Closer to the edge, the harder it will become to drum. You would need a seat belt for sure.Using RCF = 1.118 * 10-5 * r * (rpm)2 where RCF = 1G of force. A 6ft diameter drum riser spinning at 33 RPM would create just over 1G of force on the drummer. Roller coasters often exert as much as 5Gs for a reference, so the question is at what point would you no longer be able to control your limbs. My guess is about 45 RPM (2Gs of force) would be enough to make it difficult to control your arms as they moved away from your body.
I have been told on occasion that I overthink things.