Revit doesn't much care for splines from other programs...
You will need to either segment your spline to a straight-edge approximation, or try to model your spline natively in Revit.
Revit is not the greatest app for modelling high-fidelity multiple-order complex curves! That said, you should probably try using the 'Adaptive Family' template to create your geometry - you have much more control over complex curves in the 'new' 3D geometry editor than the 'standard' editor.
I have found a straight-edge approximation to be the most efficient method from a time/cost point of view - as long as my sections were correct at grid points, I didn't much care that the intervening straightline sections didn't precisely match the exact shape I wanted. In the end, I didn't even notice the facets when I rendered it, either.
If you're trying to model a boat-hull or aircraft wing though... use Rhino!
