Fiesta diffuser seems least developed to me. In comparison to Yaris or C3 it looks like being added on an already finished car...
I doubt in big effect of underbody aerodynamic on rough surfaces (that are more likely to damage it), since it needs flat surface to work together, and it works at speed, that is lower on rough roads... Even on fast gravel, I doubt in big diffusor efficiency, because body roll destroys "sealing" on the side of the floor...

I think that rear wing is still the thing aerodynamicwise.