Tickle,
There is a thread on the development of this helicopter from when it was originally being developed by Marenco:
Marenco Swiss Helicopter
My 'non aeronautical' feeling is the fuselage is quite short and bluff at its back end. I wonder how that influences flow past the fenestron when in forward flight? But I guess the same could be said of the Hughes 300, Robinson R22 and they work just fine!