The rotor can stall.
There is a video of the MD520N doing a running landing following an HV entry point. As the helicopter slides along the ground the rotor flaps back and chops off the tail boom. Most embarrassing! For a long time, most of us thought it was the pilot attempting some 'aero braking' by bringing the cyclic aft.
Turns out the cyclic was a long way forward and held there.
If you see the video, look at the rotor disk- it's progressively flapping back, due to the rotor being stalled, and even the small amount of airspeed producing retreating blade stall.
Happened to several other helicopters as well doing autorotations.
There evidently is little or no progressive warning of this happening.
The only way to figure out what the stall point is to know what the maximum thrust capability of the rotor is, and then, with some complex math, figure out the G loading, airspeed and rotor RPM at the density altitude you're doing autorotations at. I'd have to dig pretty deep in my stack of stuff to get out the calculations.
We did this when I was at National Test Pilot School for the OH-58C. Fully loaded, we climbed as high as we could until the rate of climb stopped. The rotor was complaining mightily and wouldn't take us any higher. Remember this was fully loaded - and we got to somewhere near 15K at any airspeed from 60 down to about 40 KIAS.
Back at the desk, we figured out the airspeed for the flare and touchdown, and the G loading likely in the flare (not much) and worked out the minimum rotor speed we needed to have in the flare and touchdown. Worked out to be a reasonably low number.
But the rotor will stall, so beware!
…another thing to add to the book on helicopter flight testing!! Sigh.