Maintenance of the hinges would have an effect too, I would have thought - maintainers would know more than me, but the hinges in the Robbie head can vary quite a bit in resistance to movement depending on how old they are and how well they've been looked after, in my relatively limited experience.
They don't just see-saw freely like a Jet Ranger, and I bet the relative stiffness of the flapping hinge vs the coning hinges, and the overall stiffness of individual hinges, would make for quite some variation in the patterns of movement they develop when whirling around at silly speeds.
My thoughts are that if they're properly set up and maintained and the aircraft is flown sensibly, they will work as advertised, but if they were allowed to deteriorate it could be a different story. Again, I'm not massively high hours on type, but I've done quite a bit of throwing them around (smoothly and avoiding low g scenarios) without incident, and without feeling like I was coming close to any limits. Still, you wouldn't know if the blade missed the tail boom by a millimetre or a foot, would you?
I just try to treat them like any other teetering head machine - be smooth, don't bunt and don't flog it through turbulence.