There is a substantial difference between the 'stability' of a fixed-wing aircraft and a helicopter - important for flight in IMC and the rigours of IFR.
If you look at Appendix B to Part 27 (CS or FAR) you will see that most additional regulations address stability issues.
JimL,
there are some SE helicopters with SAS and Autopilot. We had a As350b3 with sas and autopilot some years ago. I think this is not an issue. Any manufacturer would be interested to install SAS and autopilot on their SE helicopters to make them IFR capable.
H.