RW-1,
I think the question was asked, were you talking about touchdown autos at idle or shutting the engine down totally?
If the latter, nothing to add as I've only had one catastrophic engine failure, whilst sitting on the pad - gotta be happy with that.
If the former, then I can speak with some authority having done a lot of them. They are great fun and very confidence-building, but an organisation that does them regularly will have a significant number of stinger strikes (from touches to full-on digging them into the ground) or heavy, bouncy landings with the associated possibility of airframe damage. I never really cocked one up badly, but have had the odd nasty arrival with associated heartbeat waste.
This is also fine if you're not paying for the repairs, as I wasn't (well, indirectly as a taxpayer), but not really an economic proposition for the aircraft owner.
Anyway, if you get the chance, get a few into you, but there is a point of diminishing returns where if you do too many for the sport of it, just to improve the finesse part, you may have an expensively embarrassing result.
The talk above of fatalities is probably a bit overly dramatic, but as I say, it's fairly easy to bend the aircraft.
Hot tips - in a fast run-on, when you're bouncing along, freeze the cyclic in the middle; don't be tempted to pull it back instinctively in an attempt to slow yourself down. At low revs, the disc has bugger-all solidity in plane.
2. If you're doing a zero speeder touchdown, or close to, once you have levelled to the hover attitude, wait until you feel like your skids are 6 inches off the ground or so before you start using up your collective. Otherwise, the temptation is to 'milk' it all the way down, and you run out of cushioning power at the bottom.
Hope this is useful.