Fore those intrested:
This method in the revit API doesn’t work “as advertised” 
…
U can find more in info on the revitapi forum:
Solved: Re: Surprising results from Face.Intersect(face) method - Autodesk Community - Revit Products
or the building coder:
The Building Coder: Face Intersect Face is Unbounded (typepad.com)
Sad tho that after al these year this didn’t get fixed.