That's essentially right I think.

There are obviously lots of signs of ageing and erosion, and the surface survey pointed all these up - slippage from the platforms, a slight dishing on one side, an area of uncompacted soil, and scarring and deformation at the tunnel entrances and from medieval and earlier workings. But the only sudden/catastrophic events seem to have been collapses of excavation tunnels.

EH's report had a section speculating that there might be a risk of future catastrophic events arising from the deterioration of the "original" structure (eg it was essentially a POOR bit of Neolithic workmanship (!!!!!) and if the lower bit near the moat collapsed it could cause a collapse of a section of the hill!) as I mentioned, but it seemed a bit lacking in real evidence, and looked a bit silly against their contention that it's a basically robust structure. The risk of that, in our lifetimes, seems small when it hasn't happened for 4000 years! It struck me as an unjustified and unsubstantiated bit of scaremongering, for effect. Some words they could point to if there was a further internal collapse.

So in summary, I think it's fair to say that the previous major collapses, and the future risk of collapses all centre on the tunnels. (You'll appreciate this is all what I infer from what they say, I have no other source).