Ok, two cents from me as well
I once had the pleasure to talk to one of the designers of the AS 350, and one of my questions was of cource why it is so unstable. Should not have said unstable
I got an instant reply "It is not unstable, it is precise". I guess it could be argued. Anyway he said they (Eurocopter) where not really sure why exactly is was a bit twitchy (he agreed on that), a lot of contributing factors he said, no magic item.
He also said when they installed the long tailpipe on the exhaust on the B3 it became much better, and this was a surprice to them.
I have had the pleasure to fly many diffrent versions (all but the D) and in my opinion there are some diffrences.
Low skids are worce then high( I know it was argued the other way, but this is my opinion).
Short T/R output drive worce then long (standard now).
Twinstar blade with trailing edge tab much better than without.
B3 exhaust also much better.
Best contributing factor - lots of houres on the machine and to have it recent. Look on a high time 350 pilot not flying anything else - steady as a rock, but do not put that guy in a steady dampend platform, such as the S76, he will make it look bad by overcompensating