What would be the problem with a bowl of liquid?
replyThe balancing there comes from the wrist joint, not the fingers. Obviously this device is supposed to be mounted on a wrist joint that can rotate (pitch+yaw). Wrist joints are pretty well-understood these days.
Rounded handle I don't see the problem unless the hole is too small for one of the fingers.
Holding a rounded handle with a rectangular profile slider is inherently unstable as it's extremely sensitive to the pinch point.
replyAnd the bowl has a radius that make orthogonal grip impractical.