The big problem is picking the turbulence. Sometimes it will kick the s*#t out of you earlier than you thought you were going to get it, and sometimes you get none when you are sure you should get some....
Some of the Robbys that have gone down in NZ have been in the lee of the hills with 30-40 kts coming through them, i wouldn't try that in a JR or Huey, so those ones probably did mast bump, but you can't blame the machine. You hear about the aircraft gone missing, look at the weather and ask WTF were they doing in that aircraft, in that location!!
The ones that are a worry, are when they have mast bumped in what should be very calm air. There are at least three in NZ that the weather was virtually calm and they are saying it was turbulence... One particular other helicopters were in the area prior to the accident no wind, and then aircraft out searching within 20 min, no wind. Where do you get turbulence in those cases??