Practice and currency is a real must. I used to work as a training captain at a Jetbox outfit and at the same time also doing PPL training on the R22.
The AOC operator used a variety of freelance pilots. Most of the guys had oodles of experience : offshore and/or military. Most were current somewhere else : often on airliners or twin engine IFR big helos.
It wasn't my practice to do throttle chops, but "voice actuated" practice engine failures, with my hand following to deal with the twist grip. The difference between the two groups was ofetn noticeable. It was common for the experienced guys on the B206 to be slow to respond to the command and hesitant in execution. PPL students during training (who say had gone solo and were flying regularly) were fairly prompt. I guess I would not have sent them solo, if they were not.
[I should also say that everyone improved by the end of the practice which is what it is for].
The worst people were a group of PPL aircraft owners. This was before JAR-FCL, so once a PPL had his licence he never needed to fly with an instructor ever again - not possible now, and IMHO one of the (few) benefits of JAR FCL. Essentially some of this group could even be scared of doing a slow-time autorotation.
If my memory serves me, the AAIB did a study many years ago when an offshore hele crashed due to engine failure and subsequent low RRPM. They got a set of sim sessions to add an unannounced double engne failure and time the reaction time. I recall the times were pretty slow and most would not have responded in time.
These days I fly solely twin engine heles, and I can't instruct (sadly). I always used to feel sorry for line pilots struggling to keep REALLY current with just a 6 month base check and not much other oportunity to practice - now I am he!
I am contemplating doing some single engine flying in the near future, and I will be making sure that I get a very thorough workout before I am finished.
A bit of risk management is surely the practical solution (pending the arrival of the doom sensometer). In a single (and especially one with a low interia system) currency is key. At certain places I flew from, when I was hours building, autos were banned without an instructor. Clearly, this reduced the risk of problems during solo practice autos! However, I wasn't happy in that situation at all and avoided it. If any single engine pilot gets to the point where they are reluctant to do an auto, they need to get out there and in practce again with an instructor. A little game I was taught was to fly along and enter auto at the start of the next radio transmission.
When flying a single never ever let yourself be distracted (hand off the lever etc.) while in the climb or anywhere where MR pitch would give you a more rapid decline in RRPM if the donk stops. You then do the best that you can to manage the risk.