It isn't possible to do a 1 g barrel roll in a FW - it's a myth that is perpetuated, and defies the basic laws of physics and motion!
You can "attempt" a 1g barrel roll though - just won't turn out what you expected!
Aero's in a helicopter can be done "within limits" - the issue is Joe Pilot doesn't know what they are as the manufacturer or certification authority does not publish them.
Test pilots are called "test pilots" for a reason and they come in different flavours - Production and Experimental. Production Test are pretty much average Joe and Experimental Test live a bit closer to God than most!
Helicopters mostly if any do not have a large enough thrust margin to create high g loadings. They will just mush or wash off speed.
It's the other bits that most people don't think about that are the issue that maybe cannot keep up with what the aircraft "could do".
i.e. precession loads on the TR, feedback to the MR PCL's, Low or prolonged -ve g on lube, fuel and HYD systems and the list goes on.
For example - everybody knows about HYD "jack stall" in an AS350. Why is it that way? It is to give you a built in warning that you are approaching the feedback limits of the PCL's on the MR.
Dual HYD models have a LIMIT light as the warning as they will not "stall". What happens if you go past the "LIMIT"?. You probably will start bending and breaking stuff.
Operating outside the approved envelope is the territory of ETP's. Joe Pilot doing this stuff proves what exactly? You get away with it once, you get away with twice you get away with it...................................?