To set the record straight.
1. In the military we fly both SE and twin types IFR.
2. If my memory serves me, i believe that the Gazelle was the first SE IFR certificated heli(It may only have been in France, with SAS/autopilot(simplex then I believe))way back in the late 70's early 80's.
3. The Gazelles in current military service, are in the main non stabilised aircraft.Those few that are stabilised only have a rudimentary SAS.
Personally if given the option I would always prefer to fly in a twin engine aircraft, I love redundancy.
The issue is I believe, not the aircraft's capability to fly in cloud, it's more to do with what is possible (given an engine or other critical failure) to do with the aircraft if the terra firma beneath the a/c at time of failure is also in cloud - recovery options may be limited.