Hello Maxtor
Sorry for the late reply, but I didn't see your question until now.
We have used fiber gypsum from Fermacell - bought through Fredells (Stockholm). The price was almost exactly twice as expensive per m2 as single gypsum - but the fiberboards are significantly stronger than double gypsum. They deliver on their promise regarding strength.
Problems? Yes - they are much more difficult to put up compared to regular gypsum - at least if you screw them. Cutting them is absolutely not possible - a saw is a must, preferably a circular saw, but be prepared for the blade to take a beating (read - ruined!).
All in all, I am very satisfied and would choose the same material again if I had to decide.
Good luck
Per